add sis_import_id to accounts api

refs CNVS-10408

test plan
 - import accounts through SIS
 - create accounts manually
 - /api/v1/accounts/self/sub_accounts should have
   sis_import_id if you have permissions to see it
 - check that sis_import_id is in api docs

Change-Id: Ib65b3b9c5a943fa0cc134ed8346c732105d08280
Reviewed-on: https://gerrit.instructure.com/29939
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>