fix: change user from None to AnonymousUser in authenticated requests (#261)

Modified the Lti1p3ApiAuthentication authentication backend to return
AnonymousUser instead of None. This allows DarkLangMiddleware to work
properly without crashing when middleware tries to get a user info
from the request.

This investigation and patch was originally done by @OlhaShyliaieva in:
  https://github.com/openedx/xblock-lti-consumer/pull/228

I'm just rebasing it onto the latest version to land this change.