[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
I agree, but this is out of scope here. zamboni doesn't have an API exposing this right now, we'd have to implement this, and make sure this doesn't lead to privacy issues.