Fixed page editor back button sometimes going nowhere

Updated the back button to be a proper link instead of a reference to
the last viewed URL since it could break if the last page was the
current one (On validation for example).

Includes test to cover.
Also applied some styleCI changes.

Fixes #2834

app/Http/Controllers/AttachmentController.php

@@ -9,7 +9,6 @@ use BookStack\Uploads\Attachment;
 use BookStack\Uploads\AttachmentService;
 use Exception;
 use Illuminate\Contracts\Filesystem\FileNotFoundException;
-use Illuminate\Foundation\Http\Middleware\ValidatePostSize;
 use Illuminate\Http\Request;
 use Illuminate\Support\MessageBag;
 use Illuminate\Validation\ValidationException;