Add `localStorage` and `sessionStorage` to the curriculum somewhere

I'm not sure where we should add this, but MigraCode Barcelona have a workshop on the Web Storage APIs: https://github.com/Migracode-Barcelona/js2-workshop#web-storage-api

We should fold this content into the curriculum somewhere.

Relatedly: I think we should be much more clear about limitations of frontend-only-without-webstorage, frontend-only-with-webstorage, frontend+backend-without-database, frontend+backend-with-database - right now we don’t motivate these very well and our trainees can’t articulate the concepts. My experience with CYF trainees is that if you ask “What can’t we do with just a React app which we can do when we have a backend?” they will struggle to explain the answer, and I’d love for us to fix this.