Support nbgitpuller pulling from private repositories

[yuvipanda]

Description

Being able to pull from private GitHub repositories is a very commonly asked for and used feature of nbgitpuller. This was implemented for University of Toronto (utoronto-2i2c/jupyterhub-deploy#44), and is well used there (I think). To move the UToronto hub to this repo (#638), we should enable this feature generally for whatever hubs that want it.

Value / benefit

Instructors can use private git repos with nbgitpuller. I think this is a blocker for migrating utoronto as well.

Implementation details

https://github.com/yuvipanda/git-credential-helpers is the project used to enable this functionality

• Make sure it is installed in the user image
• Create a GitHub App that can be used specifically for this hub to support pulling. Some more docs need to be added on what exac
• Enable the appropriate gitconifg needed in the helm chart. https://github.com/utoronto-2i2c/jupyterhub-deploy/blob/26aa3d542bc8bcd4639dad8728484aede946f1ed/hub/values.yaml#L43 is how it is currently done on utoronto, but we should instead use https://zero-to-jupyterhub.readthedocs.io/en/latest/resources/reference.html#singleuser-extrafiles to inject this file.
• Document this publicly somewhere (blog post, docs, etc)

Tasks to complete

No response

Updates

No response

[choldgraf]

In our team meeting today I believe @yuvipanda said he'd like to give this one a shot in this cycle, so we've added it to the sprint board! We also discussed that this pattern should be documented in a blog post or something more discoverable for others, so I've added an item about that to the tasks above.