[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).
If I don't mock `build_github_urls`, I need to specify `data['content']` much more fully, it seems; otherwise, `build_github_urls` raises a ValueError. The person who wrote `test_create_file` (which runs on `github_upload_file` in _crud.py_) opted not to do that, so I followed their lead. 

I don't mind doing it, if it's an essential part of this test. Is it?