This moves the logic of setting the correct permalink values back into the controller. And it replaces the validation with a simpler one, that always works, even when the model is loaded from the DB.
Follow-up to #29634 which broke import scripts and lots of documentation on Meta.
Redesign the permalinks page to follow the UX guide. In addition, the ability to edit permalinks was added.
This change includes:
- move to RestModel
- added Validations
- update endpoint and clear old values after the update
- system specs and improvements for unit tests
