This commit updates the 'get_mentions_for_message_updates' function to
use the generic 'event_recipient_ids_for_action_on_messages' function
to determine users having access to the message and perform an
intersection with the mentioned users to filter out the users who
no longer can access the message.
It helps to add hardening such that if the invariant "no usermessage
row corresponding to a message exists if the user loses access to the
message" is violated due to some bug, it has minimal user impact.
621eb1f610