Rework the building of the recipient list on single messages.

* Deprecate bp_get_the_thread_recipients().
* Move the recipient count logic into the `members/single/messages/single.php` template.
* Introduce `bp_get_thread_recipients_count()` and `bp_get_thread_recipients_list()` to separate the logic of the deprecated function.

Props joshshashaty, Clean-Cole.
Fixes #5327.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9095 cdf35c40-ae34-48e0-9cc9-0c9da1808c22