nbgitpuller binder link generator not working with repos other than GitHub.com

### Bug description

The [nbgitpuller link generator](https://jupyterhub.github.io/nbgitpuller/link?tab=binder) makes it look like you can enter any "Git Environment Repository URL", while in fact no working links are generated for GitLab.com repositories.

#### Expected behaviour

Either a correct label such as `GitHub Environment Repository URL` or actually working links.

#### Actual behaviour

If I enter e.g. `https://gitlab.com/fkohrt/RMarkdown-sandbox` and some content repository (e.g. `https://github.com/binder-examples/r`), the resulting link starts with `https://mybinder.org/v2/gh/` instead of `https://mybinder.org/v2/gl/` and does not turn `fkohrt/RMarkdown-sandbox` into `fkohrt%2FRMarkdown-sandbox`