Ejecting from CRA, removing production minification.

[kenhoff]

Sorry for the big one!

• ejects from create-react-app, removes the react-scripts dependency and adds a whole bunch of config files.
• all of the standard yarn commands are still the same - see app/package.json
• apparently the standard UglifyJsPlugin creates invalid sourcemaps, which is why Sentry isn't giving me any line numbers to debug with.
• Removing the UglifyJsPlugin (below) and just relying on the standard webpack sourcemaps (devtool) creates correctly formatted sourcemaps, but almost doubles the bundle size to ~1mb.
• Your call if you'd like to merge this and have me focus on bugfixes, or if you'd like me to find a way to get minification correctly working.

new webpack.optimize.UglifyJsPlugin({
  compress: {
    warnings: false,
    // Disabled because of an issue with Uglify breaking seemingly valid code:
    // https://github.com/facebookincubator/create-react-app/issues/2376
    // Pending further investigation:
    // https://github.com/mishoo/UglifyJS2/issues/2011
    comparisons: false,
  },
  mangle: {
    safari10: true,
  },
  output: {
    comments: false,
    // Turned on because emoji and regex is not minified properly using default
    // https://github.com/facebookincubator/create-react-app/issues/2488
    ascii_only: true,
  },
  sourceMap: shouldUseSourceMap,
}),

[tschellenbach]

UglifyJsPlugin is pretty popular, lets have a look if we can figure out what exactly is going wrong here

[kenhoff]

Okey doke, closing until I can determine why create-react-app isn't creating valid sourcemaps.

[kenhoff]

Reopening this after a few days of research, with a couple of issues to reference:

• [HELP WANTED] Perform source-map audit against webpack webpack/webpack#3165
• Sourcemaps in chrome off by 1 line webpack/webpack#1487
• Sourcemaps Are Invalid babel/babel#6008

It appears that there's some combination of webpack, babel, and uglifyjs that's causing invalid sourcemaps to be emitted. Chrome is still able to use them (somehow, I'm not sure why - might just be with some slightly wrong mappings?), but sentry isn't picking up on them. The community hasn't been able to fix it yet, and I'm not able to dive deeper into this at this time.

This PR, if merged, will create valid sourcemaps at the expense of minification (~500kb bundle -> ~900kb bundle), and allow me to start cranking through the frontend errors on Sentry.