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/Config/app.php

@@ -182,6 +182,7 @@ return [
         'Setting'  => BookStack\Facades\Setting::class,
         'Views'    => BookStack\Facades\Views::class,
         'Images'   => BookStack\Facades\Images::class,
+        'Permissions' => BookStack\Facades\Permissions::class,
 
     ],