fix bug 939383 - modify content min-height for zone nav

https://bugzilla.mozilla.org/show_bug.cgi?id=939383

This solution isn't perfect™ in that it relies on JavaScript but it's the first effort in ensuring there's no overlap in subnav and footer.  Upon opening a subnav item, we set the min-height of the content to the subnav height.

I'll be looking over the next few days at seeing if there's a way to make the subnav in normal flow with the page (not seeing a way now), but this will fix the issue in the interim.