Bug 933404 - lightbeam home page promo

Also includes bug 928550 - webmaker home page promo

The Lightbeam promo appears on the old home page as well as the new one. The Webmaker promo is only on the new home page. Both are en-US-only on the new home page, until locales get them translated and we can remove the Mission and Mozfest promos permanently.

This doesn't include high resolution images for the webmaker promo but I'll add them in a later PR. Trying to get this out the door today and retina is an easy punt.