This commit includes several fixes and improvements to thread
original message handling:
1. When a thread's original message is deleted, the thread no longer
counts as unread for a user
2. When a thread original message is deleted and the user is looking
at the thread list, it will be removed from the list
3. When a thread original message is restored and the user is looking
at the thread list, it will be added back to the list if it was
previously loaded
37a8036b2d