Bump express from 4.17.1 to 4.17.3

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:production
...

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

package.json

@@ -7,7 +7,7 @@
     "@iarna/toml": "^2.2.3",
     "body-parser": "^1.19.0",
     "ejs": "^3.1.6",
-    "express": "^4.17.1",
+    "express": "^4.17.3",
     "express-rate-limit": "^5.0.0",
     "fa-diagrams": "^1.0.3",
     "mathjax-node": "^2.1.1",
@@ -16,7 +16,7 @@
     "prismjs": "^1.23.0",
     "redis": "^3.0.2",
     "rss": "^1.2.2",
-    "showdown": "^2.0.2"
+    "showdown": "^1.9.1"
   },
   "devDependencies": {
     "coveralls": "^3.0.4",