Configure application URLs in back-end

Relates to: stackspin!1270 (merged)

We made all application URLs configurable, this means a few things:

1. The back-end should read the ConfigMaps that contain the configured URLs. It should save these URLs somewhere (cache, DB, or maybe not cached but just reading the configmap every time?)
2. The front-end should call the back-end for linking to an app
   • This includes Hydra! So the sign-out link as well as maybe the sign in process should go through the Hydra domain configured in the CMs and available in the back-end.
   • No more hardcoded URLs in the front-end. Also no hardcoded subdomains.