MainTabs [nfc]: Prepare for and explain some FlowFixMes.

The next commit's message is quite long, so here's the motivation
for the FlowFixMes.

Importing `createBottomTabNavigator` from `react-navigation` is good
because it means more type checking! Sadly, getting that to really
work in all the ways we want won't be easy.

To sum up, we could get *part* of the way there by making tweaks to
our typed `connect` (and making sure those tweaks didn't break
something or lose type-checking elsewhere!). But there's an
additional hurdle in something that's not quite right in the
`react-navigation` libdef.

The same goes for `createMaterialTopTabNavigator`.

Greg goes into more detail at the PR thread [1].

See also discussion [2].

[1]: zulip#4114 (comment)

[2]: https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/react-navigation.20types/near/878259

src/main/MainTabs.js

@@ -1,7 +1,7 @@
 /* @flow strict-local */
 import React from 'react';
 import { Platform } from 'react-native';
-import { createBottomTabNavigator } from 'react-navigation-tabs';
+import { createBottomTabNavigator } from 'react-navigation';
 
 import type { TabNavigationOptionsPropsType } from '../types';
 import tabsOptions from '../styles/tabs';
@@ -15,8 +15,10 @@ import IconUnreadConversations from '../nav/IconUnreadConversations';
 import ProfileCard from '../account-info/ProfileCard';
 
 export default createBottomTabNavigator(
+  // We'll activate these fixmes in the next commit.
   {
     home: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: HomeTab,
       navigationOptions: {
         tabBarLabel: 'Home',
@@ -26,6 +28,8 @@ export default createBottomTabNavigator(
       },
     },
     streams: {
+      // No fixme needed; StreamTabs is a navigator from
+      // `createMaterialTopTabNavigator`.
       screen: StreamTabs,
       navigationOptions: {
         tabBarLabel: 'Streams',
@@ -35,6 +39,7 @@ export default createBottomTabNavigator(
       },
     },
     conversations: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: PmConversationsCard,
       navigationOptions: {
         tabBarLabel: 'Conversations',
@@ -44,6 +49,7 @@ export default createBottomTabNavigator(
       },
     },
     settings: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: SettingsCard,
       navigationOptions: {
         tabBarLabel: 'Settings',
@@ -53,6 +59,7 @@ export default createBottomTabNavigator(
       },
     },
     profile: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: ProfileCard,
       navigationOptions: {
         tabBarLabel: 'Profile',

src/main/StreamTabs.js

@@ -17,8 +17,10 @@ const styles = StyleSheet.create({
 });
 
 export default createMaterialTopTabNavigator(
+  // We'll activate these fixmes in the next commit.
   {
     subscribed: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: SubscriptionsCard,
       navigationOptions: {
         tabBarLabel: (props: TabNavigationOptionsPropsType) => (
@@ -29,6 +31,7 @@ export default createMaterialTopTabNavigator(
       },
     },
     allStreams: {
+      // £FlowFixMe `navigationOptions` property on component type
       screen: StreamListCard,
       navigationOptions: {
         tabBarLabel: (props: TabNavigationOptionsPropsType) => (