Created PlaylistVideo model and Playlist add function.

-Add videos to Playlist function now finished:
1. in the show.html.erb, routed the submit form to our new class PlaylistVideo with the params of a new video + hiden_field playlist_id.
2. PlaylistVideo creates the many-to-many relationship with the params from @video form.
-PlaylistVideo is routed separately (NOT nested or nesting Playlist/Videos/Comments
-PlaylistController only has create method.