Get actual gitlab/registry URLs from the gateway on renku login

[Panaetius]

blocked by SwissDataScienceCenter/renku-gateway#628

Since we currently just guess the image registry URL in renku session start, but this might break down in cases with an external gitlab, we need to get the actual gitlab and gitlab registry URLs for a deployment when doing renku login.

Once the changes are done on the gateway, we should replace our guessing code with fetching the information from the gateway.