This changes the thread header positioning of the
unread indicator to match the designs based on the route:
1. When the channel is open, show the indicator of # unread
threads with the icon
2. When the threads list is open, show no indicator since
you are on the list and can see which threads are unread
3. When a single thread is open, show the unread threads
indicator along with a left < back button, with a label
to show that this goes back to ongoing discussions
Drawer changes to come in another PR.
* Moved the settings cog from thread list to thread and
put it in a new header component
* Remove thread original message component, no longer needed
and the list item and thread indicator styles/content
will be quite different
* Start adding content (unread indicator etc.) to the thread
list item and changing structure to be more like designs
* Serialize the last thread reply when opening the thread index,
show in list and update with message bus
