Bug 868199: Update /new page.

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
Use === where appropriate.
Simplify CSS for version messages.

Switch from ID to class. Reduce nesting in LESS. Remove some redundant rules.
Define vars at top of function.

The platforms and additional_platforms are now defined with the other
function vars.
Bug 883800: Update wording on FX out-of-date.
Revert firstrun/a.html to master.

For some reason the file was accidentally changed. The copy in master is correct and includes the correct platofrm_img call which used to be part of this PR.