[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.
Looks like this is redirecting to the login page with a 401 response, so the test is incorrect. You may need to pass `auth` to your `get` call.

```python
res = self.app.get(url, auth=user.auth)
#...
res = res.follow(auth=user.auth)
```