Accept numeric xblock trace values

Self-assessment was failing in the LMS on the check between student_id:
(a) the student item model was storing it as a charfield
(b) the XBlock was storing it as an integer
This would cause the comparison to fail, resulting in a server-side error.

Since we're storing the student and item IDs as strings in the student item model, it makes sense to convert them as soon as we retrieve them from the XBlock scope ids.