This separates `ReviewCheckerChild` from `ShoppingSidebarChild` as a precursor to adding caching to the `ReviewCheckerChild` in Bug 1927956.
* The new cache functionality in the Review Checker won't apply to the previous sidebar and the complex `updateContent` method make it difficult to implement.
* Splitting up `updateContent` into smaller methods makes this a large change from the current child, but will enable implementing product caching easily in the `getProductForURI()` method.
* As both children communicate with the same `<shopping-container>` element there is a good amount of repetition in the event handlers and a few confusingly named events for the short term but this will be replacing the previous actors eventually.
Differential Revision: https://phabricator.services.mozilla.com/D232287
4a6d9ffaf8