update callback url to not include query params and give the option of configuring the url

[cianmce]

Fix Invalid redirect URI caused by query params

If a redirect_url doesn't exactly match what you have entered Spotify returns:

{"error":"invalid_grant","error_description":"Invalid redirect URI"}):

This is happening because the query params are included in the redirect_url causing it to be different each time and making it impossible to enter it beforehand on their dashboard
e.g. the following is being sent as part of the POST data in the request to https://accounts.spotify.com/api/token

redirect_uri=http://example.com/auth/spotify/callback?code=BQCXvFP5i2xtD2XG1Ns4wowvnljaXceMrfwKK9f7yapWsqaRbf_ve0iu8Ep6ssnYh3DxC-CrujAATOgrw34JHv91zpkkWKxvTDfgZvYq1k5WG0Gakw4po9wQpP0Xe6XD6EJg5GeC4JnlgmOO2nEREYwieUcw92LoTXf8ADMW9N2VNsWhBuYpFXKGxCrjretzOvxDn2BJswqFvGpkGnFwzxFDFMc3sgmUzUoIJR6BG9dF3N...

instead of just

redirect_uri=http://example.com/auth/spotify/callback

This fix is based on:
https://github.com/icoretech/omniauth-spotify/blob/a6b6ccc6ee1b0c3b90ede44efc928628f4b04420/lib/omniauth-spotify.rb#L81-L88

Another solution would be to remove this file altogether and just make omniauth-spotify a dependency