This fixes a longstanding TODO to move the contents of the
UpdateUserCountsForChannels job to the ensure_consistency!
method of Chat::Channel, which runs every 15 mins as part of
periodical updates.
This commit also addresses the performance issue of the original,
where we would fetch all channels and do an individual query to
get the count and update the count of each one. Now we do it all
in one query, and only publish the changed channels to the UI.
f6072ba804