feat: add custom-renku-url option

In renkulab, each project has two URLs: renkulab.io/projects and renkulab.io/gitlab. This may not be true for custom deployment, where the URLs may be completely different.

Until now, collect-forks assumed all deployment to follow the renkulab convention, potentially resulting in broken session start links for custom deployments. This PR adds an option to specify the base URL of the renku instance.

teach-auto collect-forks \
  --custom-renku-url https://custom-renku.demo.com \
  --group-only       https://custom-gitlab.demo.com/my-group/my-project \
  > forks.json