support minimize of modern js

There's an action to minimize javascript [using uglify-js](https://github.com/cmu-delphi/github-deploy-repo/blob/2c3e2f201fec7b96fa1e5412482b3ff510546d5b/src/actions/minimize_js.py#L22), which [only supports ES5](https://www.npmjs.com/package/uglify-js). We should use something that allows us to use more modern ES6, like maybe [terser](https://github.com/terser/terser). See this 2019 [comparison of minimizers](https://blog.logrocket.com/uglify-vs-babel-minify-vs-terser-a-mini-battle-royale/) for more info.

Start in src/actions/minimize_js.py at the existing uglify-js invocation and trace how the minimize action is run. Compare a modern JavaScript minimizer such as terser against the current ES5-only behavior, then verify that modern ES6 input is minimized successfully without breaking the deployment action.

Written by the indexing model from the issue text.