remove unused RegionBrowser.zoomCallback method
simplify some RegionBrowser sliding code
in RegionBrowser, a bit of simplifying, documenting, and removing dead code
remove compat_121.html, no longer need 1.2.1 backcompat
make Util.fillTemplate able to get values from a callback
simplify configuration loading: all query params to index.html are now directly passed as conf varrs.  also, can template conf vars in `include` urls