Groups - template file clean-up:

* Introduce missing `bp_current_group_description()` functions to match id/name/slug pairings.
* Use `groups_get_current_group()` where global touches were being used (likely more to do here.)
* Normalize `_current_group_` functions to all use the general same approach and filtering techniques.
* Add filter to the end of `bp_groups_get_action_link()` and do some general clean-up around it.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8459 cdf35c40-ae34-48e0-9cc9-0c9da1808c22