return sis pseudonyms from trusted accounts

fixes CNVS-11814

and be sure to identify the account somehow

test plan:
 * set up a trust between account A to account B
 * add a user to account B with an SIS pseudonym
 * enroll the user in a course in account A
 * course gradebook csv and quiz statistics csv should show the
   pseudonym from account B, and account B's domain
 * an API call to list students in the course shoud show the
   pseudonym from account B, and account B's domain

Change-Id: Id656d06b8119bdcb31cc3e567d49cd6ce7c8428f
Reviewed-on: https://gerrit.instructure.com/31812
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>