[ticket/12800] core.functions_display.display_user_activity.actives_after

Adding core.functions_display.display_user_activity.actives_after

https://tracker.phpbb.com/browse/PHPBB3-12800
The name of the event does not fit other events, e.g. in the same file you'll find a few examples that differ from this.
Change it to something like `core.display_user_activity_modify_actives`