Code review changes

* Replace str(self.id)
* Move PermissionMixin to base/decorators.py
* Fix import order.
* Move tests to new class. Use RemoTestCase.
* Fix import order.
* Fix test typos. Add test for PermissionMixin.
* Add active form field.
* Add uneditable activities.
* Add tab to base content under settings.
Raise error in not editable save/delete.
UI review