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

TODO:
- [x] 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
- [x] Screenshots
How is that URL generated? Is that big hash string static or dependent on the user? We probably don't want the `en-US` part and let mozilla.org figure that out.