Introduce bp_activity_admin_get_activity_actions().

This function walks through all currently registered activity actions
and returns a flattened array of the activity actions.

Format returned is:

Array(
   'activity_update' => 'Posted a new status update',
   ...
)

See #5484

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8192 cdf35c40-ae34-48e0-9cc9-0c9da1808c22