[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
It changed a couple months ago in a1775444ec0aa78ef767459d50149308caf300c1