Since we have channel message retention which deletes
messages, we can end up with cases where the thread
is still around but the message is deleted. We will
handle the cascade delete in a different commit --
for now we will ensure the thread list lookup handles
this case and doesn't error.
This commit adds an initial thread list UI. There are several limitations
with this that will be addressed in future PRs:
* There is no MessageBus reactivity, so e.g. if someone edits the original
message of the thread it will not be reflected in the list. However if
the thread title is updated the original message indicator will be updated.
* There is no unread functionality for threads in the list, if new messages
come into the thread there is no indicator in the UI.
* There is no unread indicator on the actual button to open the thread list.
* No pagination.
In saying that, this is the functionality so far:
* We show a list of the 50 threads that the user has most recently participated
in (i.e. sent a message) for the channel in descending order.
* Each thread we show a rich excerpt, the title, and the user who is the OM creator.
* The title is editable by staff and by the OM creator.
* Thread indicators show a title. We also replace emojis in the titles.
* Thread list works in the drawer/mobile.
