Replace VTCS home promo with Fx OS promo. Bug 941185.

- Replace L10n promo tag for Webmaker with 'promo_webmaker'.

- Remove all VTCS images/CSS.
Merge pull request #1539 from jpetto/bug-941185-replace-vtcs-with-fxos-promo

Replace VTCS home promo with Fx OS promo. Bug 941185.