support gitlab proxying

[williamstein]

Somebody tried to share a notebook from gitlab here:

https://cocalc.com/url/gitlab.com/sanhuoHQ/123/-/blob/main/DCGNN.ipynb

It doesn't work, but we could make it work in a similar way to github. Note that upstream https://nbviewer.org/ doesn't support gitlab, though there was a feature request to add gitlab support... in 2014! jupyter/nbviewer#371
Also jupyter/nbviewer#879

[williamstein]

Gitlab is less and less popular. Proxying url's is a major abuse vector. If anything, we should consider eliminating proxying github, not adding more abuse opportunities.