Improved UX on Mobile, New Trade Feature

This update collects a month of changes together - It's feeling a lot better on mobile, and I wanted to share the new trading feature.

This update includes a bunch of UI improvements (partly just to make things look better, and partly to behave better on mobile) and UX improvements, like pinch-zoom support, a better dragging experience, less scrolling when you don't want it, etc.

The new trade feature is something I've wanted for a while - the ability to trade excess resources for something you're missing.  The AI uses this feature, too.