fix crosslist_check course lookup by SIS ID

test plan:
 - have two courses (A and B) in the same root account
 - create a SIS ID for course B
 - for course A, go to Course Settings | Sections
   and click a section
 - press the "Cross-List this Section" button
 - in the box labeled "enter the course's ID", enter the
   SIS ID of course B and press Tab
 - you should see information about course B under
   "Selected Course", and the "Cross-List this Section"
   button should be enabled
 - the normal numeric ID of the course should work too
   (e.g., 74)
 - as should the shard-aware one (e.g., 1~74)
 - enter an invalid ID in the box and press Tab, and
   you should see a "Not authorized" message, and not a
   "Confirmation failed" message

fixes CNVS-1873

Change-Id: I82eb6bcd123267f9f7efc8168459f08fc733a8ff
Reviewed-on: https://gerrit.instructure.com/29214
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>