Bump express from 4.17.1 to 4.17.3 in /frontend

Bumps [express](https://github.com/expressjs/express) from 4.17.1 to 4.17.3.
- [Release notes](https://github.com/expressjs/express/releases)
- [Changelog](https://github.com/expressjs/express/blob/master/History.md)
- [Commits](https://github.com/expressjs/express/compare/4.17.1...4.17.3)

---
updated-dependencies:
- dependency-name: express
  dependency-type: direct:development
...

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

frontend/package.json

@@ -215,7 +215,7 @@
     "eslint-plugin-promise": "^1.1.0",
     "eslint-plugin-react": "^5.2.2",
     "eslint-plugin-standard": "^1.3.2",
-    "express": "^4.13.4",
+    "express": "^4.17.3",
     "extract-text-webpack-plugin": "^1.0.1",
     "file-loader": "^0.8.5",
     "fs-extra": "^0.30.0",