Support cmi.core.student_id/cmi.learner_id

[potapuff]

Hi to all

Is it possible to backport from raccoongang/edx_xblock commit with support of retrieving student identity? This part is very important in case with several different scorm object in on course.

https://github.com/raccoongang/edx_xblock_scorm/commit/977c0e9186d04e28f09f396370372ed56e48e13c

Sorry, I don't have enough knowledge in python and testing of EdX to submit this as a pull request.

[potapuff]

Attach pull-request with changes

[regisb]

Closed by #21