dont include UglifyJsPlugin if MINIFY_JS is false

Created by: jpdriver

Escape hatch that excludes webpack's UglifyJsPlugin if MINIFY_JS=false is set in the .env file in the project root.

Not minifying production builds was a specific client request (despite my strong recommendations!)

So I need to use this (and am doing so in a fork) -- but figured I'd open a PR in case anyone else finds this useful...

Thanks!

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Changes

• refactored the plugins array outside of the rest of the production webpack config to allow for programmatic configuration
• included new shouldMinifyJs bool that reads from process.env.MINIFY_JS
• push UglifyJsPlugin to the plugins array as normal, unless shouldMinifyJs is false

Test Steps undertaken

1. npm run create-react-app react-app from root to create a new project
2. cd react-app
3. yarn build
4. open build/static/js/main.js

5. echo 'MINIFY_JS=false' >> .env
6. yarn build
7. open build/static/js/main.js