add sis_import_id to users api

refs CNVS-10408

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

Change-Id: Id2f096548651183688d92a6f7e40c1eb2aa30b2b
Reviewed-on: https://gerrit.instructure.com/29945
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>