build(deps-dev): bump mini-css-extract-plugin in /frontend

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 1.3.5 to 1.3.9.
- [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](webpack-contrib/mini-css-extract-plugin@v1.3.5...v1.3.9)

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

Author: dependabot[bot]

frontend/package-lock.json

@@ -39,7 +39,7 @@
     "eslint-plugin-prettier": "^3.3.1",
     "file-loader": "^6.0.0",
     "html-webpack-plugin": "^4.5.1",
-    "mini-css-extract-plugin": "^1.3.5",
+    "mini-css-extract-plugin": "^1.3.9",
     "node-sass": "^5.0.0",
     "prettier": "1.19.1",
     "sass-loader": "^10.1.1",