Fix error displayed for OAuth if an existing token already matches scopes

Before this change, the OAuth system would automatically redirect to display the code if the scopes requested were a subset of those already granted by the user. However, the missing process auth resulted in `get_authenticated_user` returning None, which broke the OAuth flow.

Fixes #2131

endpoints/web.py

@@ -518,6 +518,7 @@ def deny_application():
 @param_required('client_id')
 @param_required('redirect_uri')
 @param_required('scope')
+@process_auth_or_cookie
 def request_authorization_code():
   provider = FlaskAuthorizationProvider()
   response_type = request.args.get('response_type', 'code')