add sis_import_id to enrollments api

refs CNVS-10408

test plan
 - import enrollments through SIS
 - create enrollments manually
 - GET /api/v1/courses/:course_id/enrollments
   should have sis_import_id if you have
   permissions to see it
 - check that sis_import_id is in api docs

Change-Id: I2b8e91013858ddef76d07257e9dc38d46466af91
Reviewed-on: https://gerrit.instructure.com/29941
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>