Bug 868199: Update /new page.

__Note: this should not be merged into master until localization is ready in [Bug 908297](https://bugzilla.mozilla.org/show_bug.cgi?id=908297)__

Feedback, review, and updates to this PR also happened on PR #1115.

Adds better user-agent custom messages, cleaned up images and Google
Analytics tracking.

See https://bugzilla.mozilla.org/show_bug.cgi?id=868199

 - custom messaging support for Desktop
   - non-Firefox
   - Firefox old
   - Firefox up-to-date
 - custom messaging support for Android
   - non-Firefox
   - Firefox old
   - Firefox up-to-date
 - custom messaging support for Firefox OS, informative links
 - custom messaging support for iOS, friendly unsupported message
 - improved screenshots
   - less locale-dependent
   - more platform-specific
 - added Google Analytics custom tracking
   - off-site links
   - custom var for platform reporting
 - fix media-query css for l10n-specific CSS