Babel updates: compile helpers, plugins; add plugins

[katydecorah]

This PR is part of an action item from the IE11 bug postmortem.

• Makes babel compile src/helpers and src/plugins files, these were previously copied over.
  • While rolling out 0.19.3, repos that used the version-sort helper were failing on IE11 due to an arrow function. I was able to get around it by adding dr-ui to the babelInclude in those repos.
  • This PR fixes that at the source by adding the transform arrow functions plugin.
  • We don't have a src/data dir, so that is removed fro the build script.
• Add object assign plugin to properly polyfill object.assign.
  • In replace Object.assign with a function #159 we replace an object.assign function, but this PR reverts that commit now that we are properly handling that feature.
• Clean up
  • Adding react version to eslint config to satisfy a warning from the react eslint plugin
  • Updated package.json to point at dr-ui repo rather than old frontend-documentation repo
• Adds eslint-plugin-es to fail if we use any features that we don't have babel set up to compile
  • And turns off any rules that we know to compile as expected. We can adjust these as needed or as we add more babel plugins.

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To confirm these new Babel changes are working on IE11, I did the following:

1. Ran npm run build
2. Locally installed the dr-ui build in the vector-tiles repo.
3. Ran npm start and tested in IE11 Browserstack, especially a page using the note component.

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No hurry in reviewing this PR.

[colleenmcginnis]

I tested these changes in /ios-sdk. Versions are being sorted correctly and notes look correct in IE11. @katydecorah it's up to you if you want @danswick to take a look, too!

[colleenmcginnis]

Oh wait. All works as expected on start, but I'm seeing a similar error as I was seeing in #165 when I build:

[17:37:22 Batfish] Error: Webpack compilation error.

../dr-ui/pkg/back-to-top-button/index.js
Module build failed: ReferenceError: Unknown plugin "babel-plugin-transform-react-remove-prop-types" specified in "base" at 4, attempted to resolve relative to "/path/to/folder/dr-ui/pkg/back-to-top-button"
    at Array.map (<anonymous>)

[katydecorah]

@colleenmcginnis I updated this PR with master (which now has the erroring plugin from #165 ) and confirmed:

• npm run build is successful with /ios
• site loads locally in IE 11
• site loads locally in an older browser (Firefox 44)

[colleenmcginnis]

This works as expected for me after the most recent changes! Thanks @katydecorah 💃