Refactored some core entity actions

- Created BookChild class to share some page/chapter logic.
- Gave entities the power to generate their own permissions and slugs.
- Moved bits out of BaseController constructor since it was overly
sticky.
- Moved slug generation logic into its own class.
- Created a facade for permissions due to high use.
- Fixed failing test issues from last commits

tests/SharedTestHelpers.php

@@ -80,7 +80,7 @@ trait SharedTestHelpers
      */
     protected function regenEntityPermissions(Entity $entity)
     {
-        app(PermissionService::class)->buildJointPermissionsForEntity($entity);
+        $entity->rebuildPermissions();
         $entity->load('jointPermissions');
     }