nav: Broaden type fixme to all screens in AppNavigator.

As mentioned in the commit message in a36814e which originally
added these fixmes:

  In fact we'd need fixmes on more of them... if the react-redux
  libdef didn't defeat type-checking at `connect` calls.

We're about to get a version of that libdef (plus our own wrapper)
that *doesn't* defeat type-checking at `connect`... so a bunch more
of these will need fixmes.  Just write one fixme to cover them all.

src/nav/AppNavigator.js

@@ -36,12 +36,12 @@ import LegalScreen from '../settings/LegalScreen';
 import UserStatusScreen from '../user-status/UserStatusScreen';
 
 export default StackNavigator(
+  // $FlowFixMe react-navigation types :-/ -- see a36814e80
   {
     account: { screen: AccountPickScreen },
     'account-details': { screen: AccountDetailsScreen },
     'group-details': { screen: GroupDetailsScreen },
     auth: { screen: AuthScreen },
-    // $FlowFixMe react-navigation types :-/
     chat: { screen: ChatScreen },
     dev: { screen: DevAuthScreen },
     'emoji-picker': { screen: EmojiPickerScreen },
@@ -54,7 +54,6 @@ export default StackNavigator(
     users: { screen: UsersScreen },
     settings: { screen: SettingsScreen },
     language: { screen: LanguageScreen },
-    // $FlowFixMe react-navigation types :-/
     lightbox: { screen: LightboxScreen },
     group: { screen: CreateGroupScreen },
     'invite-users': { screen: InviteUsersScreen },