Update webpack to 4.47.0 / webpack-cli to 4.10.0 / webpack-dev-server 4.15.1

[Johennes]

This updates webpack and webpack-cli to the latest 4.x version which is a stopgap towards #26229.

What works

• yarn build
  • --bail is now a config option instead of a CLI flag
• yarn start
  • -w has been removed but watching appears to work so I assume this is the new default
  • contentBase is now static.directory
  • stats is now devMiddleware.stats
  • hotOnly: true is now hot: "only"
  • --disable-host-check is now allowedHosts: "all"

Things I'm unsure about

• We've previously been overriding hotOnly: true with --hot. I think hotOnly is what we want because it auto-refreshes after build failures. Therefore, I've removed --hot
• The --output-... options seem to have been removed. In turn --output-path webapp appears to be required. webpack-cli 4.3.0 has broken auto bundling bhauman/figwheel-main#278 (comment) indicates that this might be a result of combining webpack 4 with webpack-cli 4.

---

This change is marked as an internal change (Task), so will not be included in the changelog.

[Johennes]

Given that #26229 will continue to be blocked on element-hq/compound-web#116 for at least another two weeks, I think this PR here can go ahead in the meantime. I'm adding a few more folks for review because, even though this PR is quite small, webpack is a finicky area.