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.
Add required minimum jQuery version and load in a non-conflicting manner if jQuery exists but is older than the required version.
Switch back to using $ for local jQuery alias and define a jQuery local var as well. This will prevent accidentally using the wrong version of jQuery when someone uses $ in tabzilla in the future."
Clean up conditional.
Merge remote-tracking branch 'gauthierm/bug-869530-tabzilla-old-jquery'
Merge pull request #839 from Nobox/download-tab-bugs

Bug 835506: Fix Download tab bugs
Fix bug 869538: append to lang files with unicode strings
Bug 861243 migrate manifesto page
Merge pull request #840 from retornam/bug-861243-migrate-manifesto

Bug 861243 migrate manifesto page