Fix "url not defined" error in testcenter-validation.js

[johnjohndoe]

The following pull request contain the relevant changes and discussion which led to using URL and specifying a node and a npm version.

• https://github.com/wo-ist-markt/wo-ist-markt.github.io/pull/360
• https://github.com/wo-ist-markt/wo-ist-markt.github.io/pull/361