Implement BP_Activity_Feed for activity mentions feed

This particular feed was missed during the original implementation of
BP_Activity_Feed. See #5020, r7207. As a result, the mentions feed was
attempting to load from a separate feed template file that no longer
exists. This broke the feed and filled the error logs with warnings.

Fixes #5124

Props r-a-y

git-svn-id: https://buddypress.svn.wordpress.org/trunk@7336 cdf35c40-ae34-48e0-9cc9-0c9da1808c22