Saving a user account in the UI should invoke watchdog()

Coming from #1678664: Role activity should track when changes are made to user accounts → - in order for role_activity to track changes made to user accounts, we need to have a watchdog event when user_profile_form() or user_register_form() is submitted. I was actually mildly surprised to find it wasn't already supported considering how watchdog() is generally used on all major 'entity' forms.