Add proper and tested OIDC support on the server

Note that this will still not work on the client side; the followup CL for the client side is right after this one.

oauth/services/github.py

@@ -9,6 +9,9 @@ class GithubOAuthService(OAuthLoginService):
   def __init__(self, config, key_name):
     super(GithubOAuthService, self).__init__(config, key_name)
 
+  def login_enabled(self, config):
+    return config.get('FEATURE_GITHUB_LOGIN', False)
+
   def service_id(self):
     return 'github'