Improve use of scenes in /firefox/new.

See bug 919788.

This makes a few changes:

 * if you arrive on the page directly from #download-fx the animation is suppressed.
 * dowload is initiated in an iframe so current page CSS images don't get cancelled before loading.
 * scene2 is suppressed on Android
 * fixed bug in IE < 9 going directly to #download-fx
 * improve inline commenting
 * use LESS-CSS mixin for transitions in CSS.
Open popul before sending GA tracking.

Seems to prevent popup blocker in some cases.
Redirects for /projects/devpreview for Bug 858315
Merge pull request #1275 from gauthierm/improve-use-of-scenes-on-new

Improve use of scenes in /firefox/new.
Redirect locale-specific proect pages (Bug 885856)
Merge pull request #1280 from sgarrity/bug-885856-projects-index

Redirect locale-specific proect pages (Bug 885856)
[bug 919581] Update copy on mission and about pages
Merge pull request #1279 from alexgibson/bug-919581-update-copy-on-mission-and-about-pages

[bug 919581] Update copy on mission and about pages
Merge pull request #1278 from sgarrity/bug-858315-devpreview

Redirects for /projects/devpreview for Bug 858315
Bug 916151 - home page redesign

With contributions from @pmclanahan, @alexgibson, and @pascalchevrel
Merge pull request #1269 from mozilla/bug-904696-homepage-redesign

Bug 916151 - home page redesign