icons: Add group_dm custom icon, for recent-DMs screen

Made by Vlad for mobile. This is the SVG file Vlad shared on CZO:
  https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/design.3A.20DM-conversation.20list/near/1594671

Related: zulip#119

Author: chrisbobbe

assets/icons/ZulipIcons.ttf

@@ -14,6 +14,7 @@ abstract final class ZulipIcons {
   //
   //  * Add an SVG file in `assets/icons/`,
   //    or otherwise edit the SVG files there.
+  //    The files' names (before ".svg") should be valid Dart identifiers.
   //
   //  * Then run the command `scripts/icons/build-icon-font`.
   //    That will update this file and the generated icon font,
@@ -27,29 +28,32 @@ abstract final class ZulipIcons {
   /// The Zulip custom icon "globe".
   static const IconData globe = IconData(0xf102, fontFamily: "Zulip Icons");
 
+  /// The Zulip custom icon "group_dm".
+  static const IconData group_dm = IconData(0xf103, fontFamily: "Zulip Icons");
+
   /// The Zulip custom icon "hash_sign".
-  static const IconData hash_sign = IconData(0xf103, fontFamily: "Zulip Icons");
+  static const IconData hash_sign = IconData(0xf104, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "language".
-  static const IconData language = IconData(0xf104, fontFamily: "Zulip Icons");
+  static const IconData language = IconData(0xf105, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "lock".
-  static const IconData lock = IconData(0xf105, fontFamily: "Zulip Icons");
+  static const IconData lock = IconData(0xf106, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "mute".
-  static const IconData mute = IconData(0xf106, fontFamily: "Zulip Icons");
+  static const IconData mute = IconData(0xf107, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "read_receipts".
-  static const IconData read_receipts = IconData(0xf107, fontFamily: "Zulip Icons");
+  static const IconData read_receipts = IconData(0xf108, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "topic".
-  static const IconData topic = IconData(0xf108, fontFamily: "Zulip Icons");
+  static const IconData topic = IconData(0xf109, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "unmute".
-  static const IconData unmute = IconData(0xf109, fontFamily: "Zulip Icons");
+  static const IconData unmute = IconData(0xf10a, fontFamily: "Zulip Icons");
 
   /// The Zulip custom icon "user".
-  static const IconData user = IconData(0xf10a, fontFamily: "Zulip Icons");
+  static const IconData user = IconData(0xf10b, fontFamily: "Zulip Icons");
 
   // END GENERATED ICON DATA
 }