nbgitpuller link generator not working for Launch from Canvas

Bug description

There are two issues making the nbgitpuller link generator not work for us for Launch from Canvas. Specifically,

1. the redirect is not correct, with the LTIauthenticator and Canvas the redirect part of the link should be https://HUB_URL/hub/lti/launch?custom_next=/jupyter/hub/user-redirect/git-pull/... however, the nbgitpuller link generator provides https://HUB_URL/hub/lti/launch?next=%2Fhub%2Fuser-redirect%2Fgit-pull (the jupyter part might be specific to us, see my next point, however it should be custom_next and not next)

2. We need our JupyterHub URL’s to have `/jupyter/ at the end, and the nbgitpuller link generator drop this when its included in the JupyterHub URL text box.

Specific example:

Link provided by nbgitpuller link generator which does not work:

https://course101.rudaux.com/hub/lti/launch?next=%2Fhub%2Fuser-redirect%2Fgit-pull%3Frepo%3Dhttps%253A%252F%252Fgithub.com%252FUBC-STAT%252Fstat-201-student%26urlpath%3Dlab%252Ftree%252Fstat-201-student%252Fmaterials%252Fworksheet_01%252Fworksheet_01.ipynb%26branch%3Dmaster

Link that does work (well except for the lab part - still opens to classic notebook):

https://course101.rudaux.com/jupyter/hub/lti/launch?custom_next=/jupyter/hub/user-redirect/git-pull%3Frepo%3Dhttps%3A%2F%2Fgithub.com%2FUBC-STAT%2Fstat-201-students%26subPath%3Dmaterials%2Fworksheet_01%2Fworksheet_01.ipynb&app=lab

[yuvipanda]

@ttimbers you were very close! https://github.com/jupyterhub/nbgitpuller/pull/172 has the fix. Thanks a lot for the extra poke, and am sorry this has been unfixed for so long

[ttimbers]

Update: we now can generate from Canvas Jupyter launch URLs by hand that work with Jupyter Lab, I will paste an example below. Note: this is use the ltiauthenticator with Canvas:

https://course101.rudaux.com/jupyter/hub/lti/launch?next=%2Fjupyter%2Fhub%2Fuser-redirect%2Fgit-pull%3Frepo%3Dhttps%253A%252F%252Fgithub.com%252FUBC-STAT%252Fstat-201%26urlpath%3Dlab%252Ftree%252Fstat-201%252Fmaterials%252Fworksheet_01%252Fworksheet_01.ipynb%26branch%3Dmaster

However, the nbgitpuller tool still fails to handle our hub url’s ending in /jupyter (e.g., https://course101.rudaux.com/jupyter), for example instead of the correct url above, we get the url below that is missing jupyter/ before hub:

https://course101.rudaux.com/hub/lti/launch?next=%2Fhub%2Fuser-redirect%2Fgit-pull%3Frepo%3Dhttps%253A%252F%252Fgithub.com%252FUBC-STAT%252Fstat-201%26urlpath%3Dlab%252Ftree%252Fstat-201%252Frelease%252Fworksheet_01%252Fworksheet_01.ipynb%26branch%3Dmaster