Change from manual URL construction to using a lib

Makes the code cleaner to read and more resilient to changes

Fixes https://jira.coreos.com/browse/QUAY-940

oauth/services/github.py

@@ -113,7 +113,7 @@ class GithubOAuthService(OAuthLoginService):
   def get_public_config(self):
     return  {
       'CLIENT_ID': self.client_id(),
-      'AUTHORIZE_ENDPOINT': self.authorize_endpoint().to_url_prefix(),
+      'AUTHORIZE_ENDPOINT': self.authorize_endpoint().to_url(),
       'GITHUB_ENDPOINT': self._endpoint(),
       'ORG_RESTRICT': self.config.get('ORG_RESTRICT', False)
     }