Remove hardcoded ass_clean_subject() routines.

Instead, move these instances to WP filters so they can be removed by
3rd-party plugins if desired.

This mirrors commit 19f9bdab00.
Add additional arguments to 'bp_ass_activity_notification_content' filter.
Add 'self_notify' status to ass_group_notification_activity().
Add new 'bp_ass_activity_notification_message' filter.

This is to filter the email content before sending to those subscribed
to "All Mail" or "New Topics".