This commit introduces a Chat::Publisher and MessageBus endpoint
that allows for updating a user's channel tracking state in bulk for
multiple channels, rather than having to do it for one channel
at a time.
This also required an improvement to ChannelUnreadsQuery -- now
multiple channel IDs can be passed to this to get the unread counts
and mention counts for those channels for a user, also increasing
efficiency rather than having to do a query for every individual
channel.
Followup to #20802
cab4b2cfba