Fix mobile upload page - full page not modal

- Add main_class yield to app.blade.php for proper CSS targeting
- Add upload-page-only upload-page-responsive classes to create view
- Now mobile upload (/videos/create) shows as full-width page, not modal

resources/views/videos/create.blade.php

@@ -1,6 +1,6 @@
 @extends('layouts.app')
 
-@section('body_class', 'upload-page-only')
+@section('main_class', 'upload-page-only upload-page-responsive')
 
 @section('title', 'Upload Video | ' . config('app.name'))