[Security] Bump mongoose from 4.13.21 to 5.12.5

Bumps [mongoose](https://github.com/Automattic/mongoose) from 4.13.21 to 5.12.5. **This update includes a security fix.**
- [Release notes](https://github.com/Automattic/mongoose/releases)
- [Changelog](https://github.com/Automattic/mongoose/blob/master/History.md)
- [Commits](Automattic/mongoose@4.13.21...5.12.5)

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

Author: dependabot-preview[bot]

package.json

@@ -35,7 +35,7 @@
     "express-validator": "^2.17.1",
     "http-auth": "^3.1.0",
     "jade": "^1.9.2",
-    "mongoose": "^4.1.5",
+    "mongoose": "^5.12.5",
     "morgan": "^1.5.1",
     "q": "^1.4.1",
     "sanitize-html": "^1.10.0",