New user Spotify authorisation flow

[anailis]

Initial implementation of Spotify authorisation, so that user playlists can be modified.

This application will use the authorization code flow. The user should be prompted to grant the application access via a browser upon application sign-up. This will return an API key, which should be stored in Parameter Store as a SecureString (/spotify/).

Scope required is playlist-modify-private and playlist-read-private, so that the application can read playlist contents, create new playlists, and modify existing playlists.