* avoid TypeError on when registering service workers without config

Fixes #5299

* same before config.onUpdate

* Provide callbacks in serviceWorker

* revert exporting default function

* WIP disable service worker by default (#2554)

* Updated service worker registration

* Readd default export in registerServiceWorker.js

* Updated comments about using Service Worker

* Call it serviceWorker

* Nits

* Applies dark theme and adds link to reactjs.org

* Just "learn React"

* Tweaks

* Resolved issue #2971

- changed h2 to h1 in template to adhere to accessibility guidelines.
- Added h1 styles to maintain same font size.

* Added header tag to template

Corrected templete to use semantically correct header tag for header instead of div

* Added className for App-title

- Added className="App-title" to h1 tag
- Changed CSS selector to target class instead of element

* Add service worker invalidation

* Update valid service worker check only on local host

* sw-precache-webpack-plugin, SW registration, manifest.json

* Documentation + a few SW tweaks.

* Added an unregister method, too, just in case.

* More info for the READMEs.

* Add minify to SWPrecacheWebpackPlugin config

* Fix SWPrecacheWebpackPlugin typo

* Fix file references in READMEmd

* Add instructions for testing service-worker locally

* Review feedback

* Some additional PWA metadata

* Use sw-precache-webpack-plugin v0.9.1 for node >=4.0.0 support

* Review feedback.

* Add manifest.json context in a comment.

* Fix typo

* Downgrade to sw-precache-webpack-plugin 0.9.1

* Hide changes in README until 1.0

* Hide changes in User Guide until 1.0

* Hide more docs

* mv create-react-app/index.js -> create-react-app/creteReactApp.js

* update to modern code style

* var -> cosnt

* set trailing-coma to es5 for prettier