Create Playlist model. Handles creation/updating/managing items

* Has unit tests written as well. Will become the central thing used
  for managing playlists.
Add Uniqueness Constraint to Playlist title, scoped to User.

* User cannot have two playlists with the same title.
Add Migration to change Primary key on playlist_item
vendor stuff
Add Static Method on Media to get all the media in a playlist(ordered)
Move Thumbnails to proper location
Refactor Playlists Controller. Now support editing and Deleting too

* Playlist Title and Description can be changed
* Playlists can be deleted at any time
* Playlist Controller code is now a lot cleaner.