[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
Yeah, a simple transition would work. `slideDown()` worked for me, so I wonder if there is something in our jQuery build we could remove...