Store the last "comment" message in the rooms table to compare it with the unread counter

Before a chat that was "read" but had a system message afterwards (like a user leaving)
would trigger another query to the oc_comments table to check for the unread message count,
although the "comment" counter could still be 0 as only "unreadable" messages were posted.

Signed-off-by: Joas Schilling <coding@schilljs.com>