Bump webpack (rjsf-team#1040)

* An empty string is no longer required with webpack v3

See https://webpack.js.org/migrate/3/#resolve-extensions.

* Update loaders

module.loaders is now module.rules.

https://webpack.js.org/migrate/3/#module-loaders-is-now-module-rules

Chaining loaders is no longer done using !. We only do this in one
place (style!css).

https://webpack.js.org/migrate/3/#chaining-loaders

It's no longer allowed to omit -loader.

https://webpack.js.org/migrate/3/#automatic-loader-module-name-extension-removed

We no longer need json-loader.

https://webpack.js.org/migrate/3/#json-loader-is-not-required-anymore

* ExtractTextPlugin needs a version bump

https://webpack.js.org/migrate/3/#new-extracttextplugin-options-

* Output path needs to be absolute

* NoErrorsPlugin is deprecated

* Actually upgrade webpack version

* Bump versions to webpack v4

* Move ExtractTextWebpackPlugin to MiniCssExtractPlugin for webpack v4

* Add a "mode" to each config

https://webpack.js.org/migrate/4/#mode

* This doesn't seem to be necessary any more

The documentation seems to say this is the default during prod, but I
haven't been able to make it emit any bad assets in dev either.

* This is the default in "production" mode