CSS cleanup

* Rename fireFox*.png to firefox*.png
* Change all embedded image urls to use `url(/images/..`
* Alphebetize all properties.
* Fix & Remove duplicate transition commands.
* Fix tos/pp links both turning blue when hovering anywhere over the "By proceeding" text.
* Add :active/:focus states to the tos/pp links for keyboard accessibility

fixes #450
fixes #451
fixes #452
Merge pull request #453 from mozilla/more-css-updates

CSS cleanup