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

app/Http/Controllers/UserController.php

@@ -177,7 +177,7 @@ class UserController extends Controller
         }
 
         // External auth id updates
-        if ($this->currentUser->can('users-manage') && $request->filled('external_auth_id')) {
+        if (user()->can('users-manage') && $request->filled('external_auth_id')) {
             $user->external_auth_id = $request->get('external_auth_id');
         }