1. Implement nested resources for creation and modification

In this lab, we're going to be implementing nested resources for creating and editing songs through an artist.

1. Using nested resources, set up routes and controller actions to create new song records through an artist. Hint: Don't forget to update the strong parameters.
2. Set up routes and controller actions to support editing a song as a nested resource of an artist.
3. Create a helper to display a drop-down list of artists if someone edits a song directly via /songs/:id/edit and to only display the artist's name if they are editing through nested routing. Name the helper method artist_select. Hint: You'll need to set a variable in the controller action to pass to the helper method as an argument along with a song instance.
4. Validate that new songs created for an artist via nested routing are created for valid artists, and redirect to /artists if not.
5. Validate that songs being edited via nested routing have a valid artist. Redirect to /artists if not.
6. Validate that songs being edited via nested routing are in the artist's songs collection. Redirect to /artists/:artist_id/songs if not.
7. Make sure all tests pass!