By default, webpack does not clean up dist/, so when you e.g. touch the
code and rebuild with `NODE_ENV=devel`, the old compressed assets from
a previous `production` build were left in place, and cockpit could
serve the old wrong ones. Enable the `clean` output option to fix this [1].

Also force the rewrite of all files with `compareBeforeEmit: false`, so
that the time stamps actually get updated on a webpack run. This fixes
`make` rules and unnecessary rebuilds when nothing changed.

Fixes #563

[1] https://webpack.js.org/configuration/output/#outputclean

Include CSS linting by default for our CSS/SCSS files, originally
introduced in cockpit. The stylelint configuration is copied from
760a4628282e02fbcef3.