ios native modules: Expose "AppIdentifierPrefix" (Team ID plus a dot).

Following discussion [1] about a SO answer (not currently the
accepted one) [2], we find it's not too hard, when we want the App
ID prefix (the Team ID plus a dot), to

1. Add a custom entry in ios/ZulipMobile/Info.plist that gets set to
   that string at build time, and

2. Write some minimal Objective-C to add a native module that calls
   an iOS API to grab that custom entry and exposes it to the
   JavaScript.

We followed a very clear React Native doc [3] on how to do this.

We also considered using a different doc [4] that would have helped
us package our code up very conveniently for distribution to NPM,
with a third-party tool called `create-react-native-module`. The
output of that tool seemed to have all the appropriate metadata that
we're used to seeing in modules in the React Native ecosystem, such
as a "podspec" file.

But our goal isn't to distribute this code for wide use. Still, it
would be possible to, e.g., host it on GitHub (or even locally) and
point to it from `dependencies` in our `package.json`. But this
seems like a lot more overhead than it'll ever be worth, and we
avoided doing something like that in 0b84717, an already-merged
part of issue zulip#4124, where we just wrote some Kotlin code and a few
lines in our `MainApplication.java` to import it.

We put the two new files in the same directory that has
`UtilManager.h` and `UtilManager.m`, which were also created for
Zulip-specific needs. This time, we follow the pattern of adding a
short, all-caps prefix, ZLP, as seen in

- Expo -> EX
- Unimodules -> UM
- React Native -> RCT

To link the new files, we used the Xcode UI:

1. Found the PBXGroup called "ZulipMobile" in the left sidebar. In
   Xcode 11.5, it has a folder icon. It includes the `Info.plist` as
   well as `UtilManager.h` and `UtilManager.m`.

2. Right-clicked it and selected `Add files to "ZulipMobile"...`,
   and chose those files.

3. Selected the ZulipMobile target in the project and targets list,
   and went to Build Phases -> Compile Sources to add the files
   there.

4. Saw that, surprisingly, they were already added there. So, job
   done.

Then, we tested with some in-app logging to see that we can indeed
access the App ID prefix. With NativeModules imported from
react-native, we see it at
`NativeModules.ZLPInfoProperties.appIdentifierPrefix`.

[1]: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/iOS.20native.20module/near/916750
[2]: https://stackoverflow.com/a/28714850/6792075
[3]: https://reactnative.dev/docs/0.60/native-modules-ios
[4]: https://reactnative.dev/docs/0.60/native-modules-setup

ios/ZulipMobile.xcodeproj/project.pbxproj

@@ -12,6 +12,7 @@
 		13B07FBF1A68108700A75B9A /* Images.xcassets in Resources */ = {isa = PBXBuildFile; fileRef = 13B07FB51A68108700A75B9A /* Images.xcassets */; };
 		13B07FC11A68108700A75B9A /* main.m in Sources */ = {isa = PBXBuildFile; fileRef = 13B07FB71A68108700A75B9A /* main.m */; };
 		42689E9C23466FF7007540AA /* webview in Resources */ = {isa = PBXBuildFile; fileRef = 42689E9B23466FF7007540AA /* webview */; };
+		8BEA1EF024A6983D00674040 /* ZLPInfoProperties.m in Sources */ = {isa = PBXBuildFile; fileRef = 8BEA1EEE24A6983D00674040 /* ZLPInfoProperties.m */; };
 		A148FEFC1E9D8CB900479280 /* zulip.mp3 in Resources */ = {isa = PBXBuildFile; fileRef = A148FEFB1E9D8CB900479280 /* zulip.mp3 */; };
 		C091D09FD11FDDB8E939257D /* libPods-ZulipMobile.a in Frameworks */ = {isa = PBXBuildFile; fileRef = 74F3CD22CB932FA7EEE0BB66 /* libPods-ZulipMobile.a */; };
 		CFA67D201EC23BCB0070048E /* UtilManager.m in Sources */ = {isa = PBXBuildFile; fileRef = CFA67D1F1EC23BCB0070048E /* UtilManager.m */; };
@@ -32,6 +33,8 @@
 		42689E9B23466FF7007540AA /* webview */ = {isa = PBXFileReference; lastKnownFileType = folder; path = webview; sourceTree = "<group>"; };
 		66E34CC6219226D10091B852 /* ZulipMobile-Bridging-Header.h */ = {isa = PBXFileReference; lastKnownFileType = sourcecode.c.h; path = "ZulipMobile-Bridging-Header.h"; sourceTree = "<group>"; };
 		74F3CD22CB932FA7EEE0BB66 /* libPods-ZulipMobile.a */ = {isa = PBXFileReference; explicitFileType = archive.ar; includeInIndex = 0; path = "libPods-ZulipMobile.a"; sourceTree = BUILT_PRODUCTS_DIR; };
+		8BEA1EEE24A6983D00674040 /* ZLPInfoProperties.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = ZLPInfoProperties.m; path = ZulipMobile/ZLPInfoProperties.m; sourceTree = "<group>"; };
+		8BEA1EEF24A6983D00674040 /* ZLPInfoProperties.h */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.h; name = ZLPInfoProperties.h; path = ZulipMobile/ZLPInfoProperties.h; sourceTree = "<group>"; };
 		A148FEFB1E9D8CB900479280 /* zulip.mp3 */ = {isa = PBXFileReference; lastKnownFileType = audio.mp3; path = zulip.mp3; sourceTree = "<group>"; };
 		CF6CFE2C1E7DC55100F687C7 /* Build-Phases */ = {isa = PBXFileReference; lastKnownFileType = folder; path = "Build-Phases"; sourceTree = "<group>"; };
 		CFA67D1F1EC23BCB0070048E /* UtilManager.m */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.objc; name = UtilManager.m; path = ZulipMobile/UtilManager.m; sourceTree = "<group>"; };
@@ -57,6 +60,8 @@
 		13B07FAE1A68108700A75B9A /* ZulipMobile */ = {
 			isa = PBXGroup;
 			children = (
+				8BEA1EEF24A6983D00674040 /* ZLPInfoProperties.h */,
+				8BEA1EEE24A6983D00674040 /* ZLPInfoProperties.m */,
 				3C4249EC1EF6E16500D245F1 /* ZulipMobile.entitlements */,
 				008F07F21AC5B25A0029DE68 /* main.jsbundle */,
 				13B07FAF1A68108700A75B9A /* AppDelegate.h */,
@@ -360,6 +365,7 @@
 			buildActionMask = 2147483647;
 			files = (
 				CFA67D201EC23BCB0070048E /* UtilManager.m in Sources */,
+				8BEA1EF024A6983D00674040 /* ZLPInfoProperties.m in Sources */,
 				13B07FBC1A68108700A75B9A /* AppDelegate.m in Sources */,
 				13B07FC11A68108700A75B9A /* main.m in Sources */,
 			);

ios/ZulipMobile/Info.plist

@@ -2,6 +2,8 @@
 <!DOCTYPE plist PUBLIC "-//Apple//DTD PLIST 1.0//EN" "http://www.apple.com/DTDs/PropertyList-1.0.dtd">
 <plist version="1.0">
 <dict>
+	<key>AppIdentifierPrefix</key>
+	<string>$(AppIdentifierPrefix)</string>
 	<key>CFBundleDevelopmentRegion</key>
 	<string>en</string>
 	<key>CFBundleDisplayName</key>

ios/ZulipMobile/ZLPInfoProperties.h

@@ -0,0 +1,5 @@
+#import <React/RCTBridgeModule.h>
+
+@interface ZLPInfoProperties : NSObject <RCTBridgeModule>
+
+@end

ios/ZulipMobile/ZLPInfoProperties.m

@@ -0,0 +1,24 @@
+#import "ZLPInfoProperties.h"
+
+@implementation ZLPInfoProperties
+
+RCT_EXPORT_MODULE()
+
+// For why we include this, see
+// https://reactnative.dev/docs/0.60/native-modules-ios#implementing--requiresmainqueuesetup.
++ (BOOL)requiresMainQueueSetup
+{
+  return NO; // Initialization may be done on any thread; we don't need access to UIKit.
+}
+
+- (NSDictionary *)constantsToExport
+{
+  // Items from the Info.plist.
+  return @{
+           // We've added a custom entry that gives us the App ID
+           // Prefix (a.k.a. the Team ID plus a dot). Access it.
+           @"appIdentifierPrefix": [[NSBundle mainBundle] objectForInfoDictionaryKey:@"AppIdentifierPrefix"] ?: [NSNull null],
+           };
+}
+
+@end