We use the official HTML validator for HTML validation, and we don't really use any of the htmllint features.

Also, remove popper.js from this script since it's not present in the repo.

We can achieve the same very easily.

By implementing the same approach of rollup.config.js
and replicate it in build-plugins.js, individual plugins
will display license headers.

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

Downgrade bundlesize until they make brotli optional since it doesn't work with node.js 10.x yet.

So, babel, qunit/karma-qunit and bundle-size aren't the latest versions.

Also, add a missing license header in build/workbox.js.

* Update copyright range to include 2018

* Extend copyright date range on new files from 2017 to include 2018

Also, refactor ESLint and stylelint config files; now everything will work from our root dir.

Also, include it in the `css` npm script since it's instant.