Unified firefox/new page

See [Bug 868199](https://bugzilla.mozilla.org/show_bug.cgi?id=868199)

 - [x] custom messaging support for Desktop
   - non-Firefox
   - Firefox old
   - Firefox up-to-date
 - [x] custom messaging support for Android
   - non-Firefox
   - Firefox old
   - Firefox up-to-date
 - [x] custom messaging support for Firefox OS, informative links
 - [x] custom messaging support for iOS, friendly unsupported message
 - [x] improved screenshots
   - less locale-dependent
   - more platform-specific
 - [x] added Google Analytics custom tracking
   - off-site links
   - custom var for platform reporting