When the user sends a message in a thread, we want to
create a membership for them in the background (default
to notification level of Watching) so we can track whether
they have read the thread.
Then, for now since we don't have granular message reading/
scrolling in the thread panel, we just update the thread
last_read_message_id for the user to the latest reply in the
thread when they open the thread panel. This at least will
mark the thread as read.
In future PRs we want to show the blue dot indicator in various
places in the UI for unread threads which will also require
some MessageBus functionality.
This takes into account the same issue fixed for channels
in ae3231e140
When we were deleting messages in chat, we would find all of
the UserChatChannelMembership records that had a matching
last_read_message_id and set that column to NULL.
This became an issue when multiple users had that deleted message
set to their last_read_message_id. When we called ChannelUnreadsQuery
to get the unread count for each of the user's channels, we were
COALESCing the last_read_message_id and returning 0 if it was NULL,
which meant that the unread count for the channel would be the total
count of the messages not sent by the user in that channel.
This was particularly noticeable for DM channels since we show
the count with the indicator in the header. This issue would disappear
as soon as the user opened the problem channel, because we would then
set the last_read_message_id to an actual ID.
To circumvent this, instead of NULLifying the last_read_message_id in
most cases, it makes more sense to just set it to the most recent
non-deleted chat message ID for the channel. The only time it will
be set to NULL now is when there are no more other messages in the
channel.
