Setting tab should be added to permalinks so admins do not need to have left `/permalinks`.
A new component called `AreaSetting` was added to avoid duplications and
simplify adding settings to other sections.
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
1. Use `this.` instead of `{{action}}` where applicable
2. Use `{{fn}}` instead of `@actionParam` where applicable
3. Use non-`@` versions of class/type/tabindex/aria-controls/aria-expanded
4. Remove `btn` class (it's added automatically to all DButtons)
5. Remove `type="button"` (it's the default)
6. Use `concat-class` helper
