Bump node-sass from 4.13.0 to 4.13.1 in /frontend

Bumps [node-sass](https://github.com/sass/node-sass) from 4.13.0 to 4.13.1.
- [Release notes](https://github.com/sass/node-sass/releases)
- [Changelog](https://github.com/sass/node-sass/blob/master/CHANGELOG.md)
- [Commits](sass/node-sass@v4.13.0...v4.13.1)

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

Author: dependabot[bot]

frontend/package-lock.json

@@ -27,7 +27,7 @@
     "eslint": "^5.16.0",
     "eslint-plugin-prettier": "^3.1.1",
     "eslint-plugin-vue": "^5.0.0",
-    "node-sass": "^4.12.0",
+    "node-sass": "^4.13.1",
     "prettier": "^1.19.1",
     "sass-loader": "^8.0.0",
     "vue-template-compiler": "^2.6.10"