Add option to show false-color probability on sky map

[lpsinger]

I would be cool to be able to see either the contours or the actual false-color map for localizations in the sky map viewer. I have a fast in-browser HEALPix viewer that I wrote years ago for the "First Two Years" paper data release. It uses WebGL and Three.js. It could even be adapted to display arbitrary HiPS survey images. This could be spun off as a separate project for a very responsive in-browser HEALPix image viewer.