R1:59949bf81150

Add "device" JS/CSS for reactive stuff

Summary:
As we work through @chad's redesign, one thing I want to do is improve the tablet/mobile experience.

Add a "device" behavior which sets a "device-phone", "device-tablet" or "device-desktop" class on the root div. The behavior (device names, width triggers) is mostly based on Bootstrap.

Also adds a preview viewport=meta tag, which makes the iPhone not scale the page like crazy and is a desirable end state, but currently makes the app less usable since…