Created tests for JunctionRepo class.

-JunctionRepo class should be able to add the project_id and song_id to the junctions table in songify-db.
-project_id uses key :id because the project_id comes from the URL and as a result, is represented as id in params.
Completed JunctionRepo class + Minor changes to Playlist related classes/tests.

-JunctionRepo class is the junction table for playlist and songs. The following methods have been included:
1. Creates junctions table (id, playlist_id, song_id) in songify-db
2. Drops jucntions table and find method for testing purposes
3. Adds a row to the junctions table, with params as input
4. Finds all songs associated with the playlist_id (input as part of params) and outputs an array of Song instances.
-Deleted the song property in all Playlist related files. Now Playlists should have three properties (id, name, description)
-Made all necessary pathing adjustments including adding JunctionRepo in repo_helper
-JunctionRepo passed all tests.
Rendered '/playlist' +'/playlist/:id', which show playlists and its songs.

-get request to '/playlist' shows all playlists. post request adds a playlist and redirects to '/playlist'.
-get request to '/playlist/:id' shows all songs for that playlist (:id = playlist id). post request adds a song to the playlist and redirects to '/playlist/:id'.
-Added method find_all to SongRepo for the purpose of displaying all songs for the seletion option in each '/playlist/:id' pages.
-Each page passed the test of adding and showing the playlist/songs.