Firefox /firstrun A/B tests. Bug 865433.

- Abstracted simple footer from /firefox/new/

- Added L10n strings block to templates/base.html

- Fixed L10n comment case typo on /firefox/partners/index.html

- First crack at general modal JS (media/js/mozilla-modal.js)
Merge pull request #867 from mozilla/bug-865433-first-run

New /firefox/firstrun/ views