Reinstate bbPress' topic subscription in BuddyPress groups.

If the current user is not subscribed to the group's 'All Mail' option,
allow the user to use bbPress' built-in topic subscription functionality.

This emulates GES' old 'Follow / Mute' feature.  (Note: This is managed
by bbPress and *not* by GES.)

If the current user is subscribed to the group's 'All Mail' option, we
disable this bbP's functionality to prevent duplicate emails.
Revert "Temporarily remove 'Group Forum' notification settings if using bbPress 2."

This reverts commit 1cff0905d15c342e41d35bfb801a04fa9b879e8a.
Disable certain notification options if using the bbPress plugin.

Specifically, remove:
- Replies in a forum topic you've started
- Replies after you in a forum topic

Introduce helper function - ass_get_forum_type() - to determine what forum
type is active.