allow capa problems access to anonymous_student_id

An upcoming MITx course will be using jsinput to embed custom response assessments from an external server.  That server needs to know the anonymous id of the student doing the assessment.  

This PR allows capa problems access to the `aonymous_student_id`, via the usual capa context variable substitution method, i.e. `$anonymous_student_id`.  This is analogous to how the HTML module makes a substitution for `%%USER_ID%%`, but consistent with capa problem semantics.

Unit test provided.

@chrisndodge ?