WIP: Hackathon: implement user service to return currently-logged-in user

This depends on: https://github.com/edx/XBlock/pull/273/files
Hi @nedbat.  The imports are inside the `try...except` b/c the `DjangoXBlockUserService` eventually depends on django, so I put that in `common/djangoapps`, but that means the xmodule library tests can't import them.  This is something like the rationale in https://github.com/Stanford-Online/edx-platform/blob/jbau/xblock-user-service/common/lib/xmodule/xmodule/modulestore/django.py#L157