Load locales files from the CDN when in a web context (bug 1079894)

https://bugzilla.mozilla.org/show_bug.cgi?id=1079894
We could do without if we only had the package - it doesn't need any of the data-attributes set on `body` (well, it uses data-languages, but we could remove that if we wanted).

It's useful for the website though, to share stuff with zamboni without having to require settings. It's especially important for `l10n.js` which has to be loaded before include.js and therefore before settings.