[feature] GitHub CRUD tests

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.

This closes [PR 1278](https://github.com/CenterForOpenScience/osf.io/pull/1278).
Actually, `Github.from_settings` also has the possibility of sending an API request: https://github.com/csheldonhess/osf.io/blob/258da94323d605282358e964459e2a5671e5498f/website/addons/github/views/crud.py#L105. This will also need to be mocked.