Add "Download Firefox" Facebook tab

Adds "Download Firefox" Facebook tab under /facebook/downloadtab/ URL. The tab revolves around a download button (using the new download_firefox function) but also has Facebook social integration after the download in the form of timeline shares and app invites. The third screen has cross promos for Affiliates and Facebook messenger for Firefox. There's a slightly tweaked layout of the Download Tab available under /facebook/downloadtab/noscroll/ with fixed 800px height so that the Facebook tab iframe doesn't show a vertical scrollbar.

The Download Tab requires two local settings for its Facebook integration: FACEBOOK_PAGE_NAMESPACE, the username/namespace of the Facebook page in which the tab is to be installed, and FACEBOOK_APP_ID, the app ID for the Download Tab Facebook app.

Other additions include support for decorators in mozorg.utils.page, adding a bare template for URLs like this one that aren't part of the main mozilla.org site, server-side redirects for the top frame via small JavaScript template and other Facebook-related utilities.

Fixes bug 835506.
Merge pull request #666 from Nobox/facebook-tab-download

Download Firefox Facebook Tab