Cocoa Market - Responsive Design Tweaks

[cbujara]

This simulation would work best if students could interact via smartphone, which would facilitate recording of trades after face-to-face negotiation. Some design elements require tweaks to work well on a mobile device.

[cbujara]

Known smartphone issues (tested on iPhone 6 Safari and Chrome): unable to resize/close instructions displayed on launch. Same with error messages. Trade amount slider a bit clunky. We may discover more issues once we get past these. Seems like it should mostly be okay, though.