This commit adds a tracking dropdown to each individual thread, similar to topics,
that allows the user to change the notification level for a thread manually. Previously
the user had to reply to a thread to track it and see unread indicators.
Since the user can now manually track threads, the thread index has also been changed
to only show threads that the user is a member of, rather than threads that they had sent
messages in.
Unread indicators also respect the notification level -- Normal level thread tracking
will not show unread indicators in the UI when new messages are sent in the thread.
This commit adds the thread index and individual thread
in the index list unread indicators, and wires up the message
bus events to mark the threads as read/unread when:
1. People send a new message in the thread
2. The user marks a thread as read
There are several hacky parts and TODOs to cover before
this is more functional:
1. We need to flesh out the thread scrolling and message
visibility behaviour. Currently if you scroll to the end
of the thread it will just mark the whole thread read
unconditionally.
2. We need to send down the thread current user membership
along with the last read message ID to the client and
update that with read state.
3. We need to handle the sidebar unread dot for when threads
are unread in the channel and clear it based on when the
channel was last viewed.
4. We need to show some indicator of thread unreads on the
thread indicators on original messages.
5. UI improvements to make the experience nicer and more
like the actual design rather than just placeholders.
But, the basic premise around incrementing/decrementing the
thread overview count and showing which thread is unread
in the list is working as intended.
