Revert the naming of default view `context` argument

@cpennington I was reading through your pull request on edx-platform and noticed in response to a request to change the unused argument `context` to `_context` you replied:

> If I do that, then I'm changing the signature from the standard for a view, which means that a call using a named argument view_fn(context=blarg) wouldn't work.

I realized I had done just this in my pep8/pylint cleanup on XBlock. This reverts that behavior.