Added video filtering function to Playlist.

-Added an if statement with the following logic:
1. if the video you are trying to add to the playlist already exits AND if there isn't a relationship between that playlist and video in the joint table, then creat the relationship
2. else create the video and create the relationship between video and playlist.
-Changed the :id to :playlist_id for the purposes of finding the video's relationship to playlist in our joint table (the column's label is :playlist_id not :id)
-Passed all test trials.