Bug 835506: Fix Download Tab share image URL

Adjust to production server's CDN media URL. Facebook dialogs always need absolute URLs so to make it work in all environments, whatever the media URL is missing must be prepended.
Bug 835506: Fix Download Tab analytics tracking

Wasn't correctly tracking the custom URL.
Merge pull request #839 from Nobox/download-tab-bugs

Bug 835506: Fix Download tab bugs