Bump express from 4.15.3 to 4.17.3

Bumps [express](https://github.com/expressjs/express) from 4.15.3 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/4.15.3...4.17.3)

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

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

package.json

@@ -30,7 +30,7 @@
     "copy-webpack-plugin": "^4.0.1",
     "css-loader": "^0.28.0",
     "eventsource-polyfill": "^0.9.6",
-    "express": "^4.14.1",
+    "express": "^4.17.3",
     "extract-text-webpack-plugin": "^2.0.0",
     "file-loader": "^0.11.1",
     "friendly-errors-webpack-plugin": "^1.1.3",