[WIP] Add new newsletter for desktop / mobile (bug 1109250)

TODO:
- [ ] Add separate view on mobile, accessible from the account settings page (re-using template and events handling code)
- [ ] Better styling of the account settings page
- [ ] Add language list
- [ ] Screenshots
`/newsletter-signup` should be added to [`routes.js`](https://github.com/mozilla/fireplace/blob/master/src/media/js/routes.js).