[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.
It's created with a bunch of mocked stuff, so, like you'd guess, it generates a 401 (even if I swap out localhost:80 for localhost:5000). 

Here's an example of res.location: http://localhost:80/login/?next=/project/k9zhq/github/file/my_file/

I could mock-follow it, but I think that's outside the scope of this test.