feat: unify GitHub App token auth into materialize_repo, removing separate github_import command

[Scott McMaster (scottmcmaster)]

Summary

We had two different ways of doing git import -- the one I added that supports more extensive features in terms of git ref and letting you set up your nix config to a subdirectory which relied on ssh agent keys, and the one cooper (@czxtm) added that supported getting a token but none of the rest of the stuff I worked on last week. I tried to merge these into one great uber-feature.

Note that this is stacked on "onboarding" and there's a TODO to change the UI to support refs and subdirs. This one just supports owner/repo.

Test Plan

• No test plan needed

Docs

• Docs updated (companion PR in darkmatter/nixmac-web: #___)
• No docs update needed

[Scott McMaster (scottmcmaster)]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• feat: unify GitHub App token auth into materialize_repo, removing separate github_import command #445 👈 (View in Graphite)
• Onboarding #440
• develop

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