Ensure that the 'Comments' dropdown filter on the activity directory includes new- and old-style comment activity

The change in BuddyPress 2.0 to storing blog comment activity in
activity_comment items resulted in the 'Comments' dropdown filter breaking: it
continued to return only 'new_blog_comment' items. This changeset introduces
a number of filters to ensure that both the appropriate 'activity_comment'
and 'new_blog_comment' items are included when passing 'action=new_blog_comment'
to bp_has_activities().

See #5608

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8747 cdf35c40-ae34-48e0-9cc9-0c9da1808c22