build(deps-dev): bump mini-css-extract-plugin in /frontend (#749)

Bumps [mini-css-extract-plugin](https://github.com/webpack-contrib/mini-css-extract-plugin) from 1.3.1 to 1.3.4.
- [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@v1.3.1...v1.3.4)

Signed-off-by: dependabot[bot] <[email protected]>

Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

frontend/package-lock.json

@@ -39,7 +39,7 @@
     "eslint-plugin-prettier": "^3.1.0",
     "file-loader": "^6.0.0",
     "html-webpack-plugin": "^4.0.1",
-    "mini-css-extract-plugin": "^1.0.0",
+    "mini-css-extract-plugin": "^1.3.4",
     "node-sass": "^5.0.0",
     "prettier": "1.19.1",
     "sass-loader": "^10.1.1",