Clean up `bp_activity_can_comment()` (as it's seen a few tweaks over the past few years) to use one `switch` statement based on `bp_get_activity_action_name()`.

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8735 cdf35c40-ae34-48e0-9cc9-0c9da1808c22