Support pulling from private git repos

[yuvipanda]

Currently, only pulling from public git repos is supported.

We should also support pulling from private git repos.

Ref jupyterhub/nbgitpuller#53

[yuvipanda]

Let's start by:

1. Creating a bot user associated with this JupyterHub
2. This bot would be given access to private repositories that need to be pulled by users on this JupyterHub
3. We authenticate all users from this JupyterHub as that bot user account, so they can pull these private repositories.

Should be fairly easy to do. However, it gives all users on this JupyterHub access to these private repo, not just any given set of users. jupyterhub/jupyterhub#2245 should help with that once it lands.

[yuvipanda]

/cc @ntaback

[yuvipanda]

I've instructions at https://github.com/utoronto-2i2c/jupyterhub-deploy/#pulling-from-private-github-repos-with-nbgitpuller. However, the link generator has a bug that prevents folks from using it to generate links - I've a PR (jupyterhub/nbgitpuller#163) to fix that. Once that's merged, nbgitpuller.link should work for this too.

In the meantime, check out https://jupyter.utoronto.ca/hub/user-redirect/git-pull?repo=git%40github.com%3Autoronto-2i2c%2Fprivate-repo-test&urlpath=tree%2Fprivate-repo-test%2F&branch=main where you can see a private repo being pulled.