* Simplify Okta assignment statuses.
The Okta assignment statuses have been simplfieid so that there's only a
status on the Okta assignment object itself and the number of states is
reduced.
* Fix tests, remove unused code.
* Add in Duration for update statuses to prevent changing status too soon.
* Make sure LastTransition is UTC.
* Remove actions in favor of targets, make cleanup time non-nullable.