nbviewer preview doesn't work for branches with / in them

[yuvipanda]

Bug description

When accessing https://mybinder.org/v2/gh/Darshan808/jupyterlab/feature%2Fworkspace-indicator?urlpath=lab, I see:

The iframe URL is https://nbviewer.jupyter.org/github/Darshan808/jupyterlab/tree/feature%2Fworkspace-indicator, which does not work because it gets redirected to https://nbviewer.jupyter.org/github/Darshan808/jupyterlab/tree/feature/workspace-indicator/ - note the trailing slash gets added, but we lose the %2F. However https://nbviewer.jupyter.org/github/Darshan808/jupyterlab/tree/feature%2Fworkspace-indicator/` does work.

So I think we can fix this by just adding the trailing slash ourselves, although this is also bug in nbviewer itself.