Sort activity actions when they're added, instead of when they're fetched.

Sorting activity actions can be an expensive process, so should not be done
any more frequently than necessary. And it's only necessary to sort when a
new action has been added. So instead of sorting in bp_activity_get_actions(),
we now do it in bp_activity_set_action().

Props boonebgorges, imath.
Fixes #6865.