Wrap all alert payloads in useMemo to avoid infinite re-rendering (#182)

* Wrap all alert payloads in useMemo to avoid infinite re-rendering

This manifested in production as a browser freeze for any
user who saw the 15%-off-to-upgrade message.

TNL-7400

* fixup! meant to say identity, not equality

src/generic/user-messages/hooks.js

@@ -8,6 +8,17 @@ export function useAlert(isVisible, {
   const { add, remove } = useContext(UserMessagesContext);
   const [alertId, setAlertId] = useState(null);
 
+  // Please note:
+  // The deps list [isVisible, code, ... etc.] in this `useEffect` call prevents the
+  // effect from running if none of deps have changed. However, "changed" for objects is
+  // defined in terms of identity; thus, if you provide a payload that is *seemingly* equal
+  // to the previous one but *actually* a different object, then this effect will run.
+  // If you are particularly unlucky, this will cause an infinite re-render loop.
+  // This manifested itself in TNL-7400.
+  // We hope to address the underlying issue in TNL-7418.
+  // In the mean time, you may follow the pattern that `useAccessExpirationAlert`
+  // establishes: memoize the payload so that the exact same object is used if the
+  // payload has not changed.
   useEffect(() => {
     if (isVisible && alertId === null) {
       setAlertId(add({