[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).
Oh, I see the rationale now.

It looks like the only thing missing from the `dict` passed to the `Content` constructor is the `type` item, which is used in `build_github_urls`:  https://github.com/csheldonhess/osf.io/blob/258da94323d605282358e964459e2a5671e5498f/website/addons/github/api.py#L320 . Therefore, this should do the trick:

```python
        github_mock.update_file.return_value = {
            'commit': Commit(dict(sha=sha)),
            'content': Contents(dict(size=size, url='http://fake.url/', type='file')),
        }
```