Added custom playlist functionality.

-On '/', added form field and submit button (index.erb + index.css). Located bottom left. Typing playlist name and submitting sends get request to '/playlist'.
-On '/playlist', added name(value prefilled from params), songs, description fields, and submit button. Also displays table with all playlist from playlists table in songify-db (playlist.erb).
-Routing implemented in server.rb for get/post request. post request redirects to '/playlist' and get pushes to playlist.erb.
-Clicking on the playlist name of playlist table sends a get request to '/playlist/:id'. Will be used to view/add songs in the playlist.