Migrate from uglify to terser

[bjankord]

Feature Request

Description

The react team has identified that uglify-es breaks the new React build (16.6) due to bugs in uglify-es.

Currently, we use uglify via uglifyjs-webpack-plugin. We should switch over to Terser, a fork of uglify-es that is maintained and being promoted by the react team. Webpack also looks to be switching to terser.

We'll want to swap out uglifyjs-webpack-plugin for terser-webpack-plugin