[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).
When I mock from_settings, I pretty much get the first two of these for free, because some very thoughtful person set up `github_mock` to return fake repos and contents really nicely. I have to change how I mock `connection.history`, but that's fine.