Activity: Combine the @mentions enqueuing function.

Previously, there was a function for loading in both admin and frontend contexts.
I've combined the loading into one function which removes a bunch of duplicated logic, probably left over from an early approach at building the suggestions feature.

See #3278

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8773 cdf35c40-ae34-48e0-9cc9-0c9da1808c22