frontend, api: allow custom emails as commenters

I'm really sorry this came out as one huge commit, but I'm afraid
I can't split this up.

Closes https://gitlab.com/commento/commento-ce/issues/9

api/oauth_google.go

@@ -37,5 +37,7 @@ func googleOauthConfigure() error {
 		Endpoint: google.Endpoint,
 	}
 
+	configuredOauths = append(configuredOauths, "google");
+
 	return nil
 }