[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 would be nice if the user could opt out from the __Account Settings__ page à la [app recommendations](https://bugzilla.mozilla.org/show_bug.cgi?id=1072637).