Follow-ups to thread support, plus additional features

[kevinaboos]

• In mobile view mode, use the StackNavigation widget's new feature to push a new view instance onto the stack, such that the back button will go back to the main room timeline's RoomScreen, not the root view (RoomsList).
• Add a new option to the MessageContextMenu: "Reply in Thread", and support creating a new thread with the first threaded reply.
• Show a header at the top of a thread-focused timeline that informs the user that they're viewing just a thread in a particular room, not the entire room.
  • In the future, we can also add a button to jump back to the original "thread root" message in the main room timeline, but we don't yet support navigating to specific events in a room.