MDL-48720 mod_forum: Only copy discussion preferences when moving posts
Rather than adding discussion preferences to a user based upon their
subscription to the source forum, only respect any actively selected
preference. That is to say that:
* if a user has opted into a discussion and following a move would be
unsubscribed, re-subscribe them;
* if a user has opted out of a discussion and following a move would be
subscribed once more, unsubscribe them; and
* if a user has not made an active choice, follow the discussion preference
for the target forum.