Upgrade to webpack 4

[spalger]

In order to make working with code splitting and async imports easier in #16253, I've gone ahead and upgraded to webpack 4. This seems to offer a significant advantage in build times (about 20 sec less when starting with no caches) and is working great locally. The primary changes involve the removal of the extract-text-webpack-plugin, and the CommonsChunkPlugin. The functionality of both has been preserved though, switching instead to the mini-css-extract-plugin and using webpack's new built in chunking configuration.

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[spalger]

Okay, I might have spoken too soon, I'm seeing very slow rebuild times and OOMs in rebuild... Will take another look tomorrow.

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[spalger]

Just did a little research, our current module counts by category are:

node_modules: 4143
ui/public: 1016
core_plugins: 806
x-pack: 1519
ui_framework: 147
webpackShims: 10
synthetic: 40
other: 36
total: 7717

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[elasticmachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request

[spalger]

We've found a way around needing webpack 4 in #16253, and with the performance issues and the fact that @mistic is also working on improvements to the optimizer that kind of intertwine with this effort... I'm going to close for now.