Add brackets to comparison in `bbp_is_single_forum()`.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5485 9866e705-20ec-0310-96e7-cbb4277adcfb
Pass `$topic_id` and `$forum_id` to relevant favorites and subscriptions filters. Hat tip DJPaul. Fixes #2651.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5486 9866e705-20ec-0310-96e7-cbb4277adcfb
Dedicated filters to allow manipulating of users who will receive subscription emails. Hat tip roysmyth. Fixes #2627.

git-svn-id: https://bbpress.svn.wordpress.org/trunk@5487 9866e705-20ec-0310-96e7-cbb4277adcfb