[feature] GitHub test stubs for update, delete, and view

I filled in the test stubs for github_update_file, github_delete_file, and github_view_file.

This replaces [PR 1243](https://github.com/CenterForOpenScience/osf/pull/1243) in the old repo, which I am going to close now.
Is it necessary to mock `build_github_urls`? I don't see it being used in `github_view_file.`