Fixed footer ads again

[davidfischer]

This change allows displaying a fixed footer promo ad.

• This is an alternative ad to the sidebar and existing footer ads. It would be instead of a sidebar or footer ad and not in addition to another ad
• It is for mobile devices only.
• It supports all themes (but is only on for the themes with ads already)
• It only supports text (no images).
• vendors a JS browser detection library to detect mobile. This adds ~7KB to our bundle which should be less than ~1kb gzipped.
• a side effect is that it needed to push up the version selector flyout menu so they aren't obscured.

[ericholscher]

Looks simple enough. I'd be up for testing this with a community ad in the near future.