Switch to using UglifyJS for JavaScript builds.

UglifyJS is a more dependable minifier that does a better job at
compressing JavaScript files. We have been using a Python fork of jsmin,
which isn't being actively developed, and has had some bad releases that
has broken us. With UglifyJS, we can be more confident in stable
releases from here on out.
Release Review Board 1.7.18.
Merge branch 'release-1.7.x'