Student Notes: cleanups.

@jmclaus , @tymofij please review.
> are you sure that self.runtime.anonymous_student_id is not course-specific?
> 
> get_id_token is designed to pass non-specific anonymous_user_id, so if self.runtime.anonymous_student_id gets specific, they will not match.

Looks like it uses the same (method)[https://github.com/edx/edx-platform/blob/anton/edxnotes-cleanup/lms/djangoapps/courseware/module_render.py#L499] that we're using before:  https://github.com/edx/edx-platform/pull/6322/files#diff-651c4d4682a9c8735479de2667d9dbddL5