Bug 971618 - Add support for OMA DRM direct downloads

I didn't fix all the lint errors on download.js on this patch cause it would dillute the changes. Can do that on a continuation bug.
Is there some reason you're not using data-l10n-id here?  If you do it this way, I think the user might see a flash of english and then see the localized text, depending on when the hidden attribute is changed. I think it would be simpler to just localize it statically with data-l10n-id.