WIP: Use UglifyJS to minify JavaScript in the LMS

Possible short-term fix until we get RequireJS optimizer set up.

I tested this change in a sandbox and saw these improvements (compared to prod):

* lms-main_vendor: 382 KB --> 253 KB
* lms-application: 15.1 KB --> 9.7 KB
* lms-modules: 207 KB --> 134 KB
* lms-courseware: 2.3 KB --> 1.9 KB
* discussion: 28.7 KB --> 23.6 KB

For those files, that's about a 33% decrease in file size.

@andy-armstrong @polesye Thoughts?