Add querystring test URL for fx firstrun. Bugs 901028, 909943.

- Remove old A/B firstrun test URLs

- Remove B firstrun template

- Remove GA experiment code from /firefox/firstrun/

- Add GA experiment code to /firefox/new/
Add tests and improve a little pep8-iness.
Add visual design guidelines for Firefox OS
Bugs 870998 872747 8873482 872747 884424 863160
Merge pull request #1034 from sgarrity/bug-870998-firefoxos-type

Add visual design guidelines for Firefox OS
at2x retina image function - refs bug 898013

Mozilla.ImageHelper class that handles detecting high-res display and also handles showing high-res images as well as handling platform images
Merge pull request #1119 from nburka/bug-898013-retina-img-tag

at2x retina image function - fix bug 898013
Fix bug 914254: Fix less compile errors due to missing variables.

* Add buttons related variables for non-responsive styles
* Same variables introduced into responsive lib.less in Bug 859427
Localize placeholders in privacy contact form
Merge pull request #1210 from flodolo/placeholders

Localize placeholders in privacy contact form
Merge pull request #1207 from jpetto/bug-909943-firstrun-test-querystring

Add querystring test URL for fx firstrun. Bug 909943.