Bump mini-css-extract-plugin from 2.8.1 to 2.9.0

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 2.8.1 to 2.9.0.
- [Release notes](https://github.com/webpack-contrib/mini-css-extract-plugin/releases)
- [Changelog](https://github.com/webpack-contrib/mini-css-extract-plugin/blob/master/CHANGELOG.md)
- [Commits](https://github.com/webpack-contrib/mini-css-extract-plugin/compare/v2.8.1...v2.9.0)

---
updated-dependencies:
- dependency-name: mini-css-extract-plugin
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <support@github.com>

package.json

@@ -32,7 +32,7 @@
     "jquery-ujs": "^1.2.3",
     "jstree": "^3.3.16",
     "keycharm": "^0.4.0",
-    "mini-css-extract-plugin": "^2.8.1",
+    "mini-css-extract-plugin": "^2.9.0",
     "moment": "^2.30.1",
     "opensans-webkit": "^1.1.0",
     "pnp-webpack-plugin": "^1.7.0",