Implemented polymorphic relationship of comments with videos and playlist.

-Changed the model structures of comments, videos, and playlist to include polymorphic => true and the relationships
-Migration: added commentable_id and commentable_type columns to comments
-Changed the routing structure so that playlist nests comments
-The comments form now has hidden field of commentable_id and commentable_type to be used for CommentController's create method.
-CommentController's create method changed to take in a comment create request from any of video/playlist/comments
-Also changed the redirect route of the CommentController's create method to account for the above change.
-Both Video and Playlist now renders all the comments of that specific Video/Playlist.