MDL-48221 mod_forum: Improve discussion subscription on post

If the user has posted in the forum before, follow their previous
discussion preference.

If the user has not posted before, but is subsscribed to the forum, follow
their forum preference.

Otherwise, follow their user-preference.

This has the result that if a user has unsubscribed from a forum and writes
their first response; the user preference will be followed. This may
override the forum subscription.

This commit also changes from a dropdown to a checkbox in both the post
form, and the user profile.