[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
It's there https://github.com/mozilla/fireplace/pull/843/files#diff-f9cf4d502996d541c449ed3e7575f889R27.