Update way of checking for registration disabled

Spec says 403 + M_FORBIDDEN

src/components/structures/auth/Registration.tsx

@@ -223,7 +223,7 @@ export default class Registration extends React.Component<IProps, IState> {
                 this.setState({
                     flows: e.data.flows,
                 });
-            } else if (e.httpStatus === 403 && e.errcode === "M_UNKNOWN") {
+            } else if (e.httpStatus === 403 || e.errcode === "M_FORBIDDEN") {
                 // At this point registration is pretty much disabled, but before we do that let's
                 // quickly check to see if the server supports SSO instead. If it does, we'll send
                 // the user off to the login page to figure their account out.