add root account filter to course index API

note: this endpoint is subject to change and not yet documented

test plan:
 1. enroll a student in courses in two different shards
 2. use the "List your courses" API endpoint
    against both shards' root account domains
    - all courses in both shards should be returned
 3. use the "List your courses" API endpoint with
    current_domain_only=true
    ( /api/v1/courses?current_domain_only=true )
    - each call should return only the courses
      in the shard of the domain root account
      you are hitting
 4. use the "List your courses" API endpoint with
    ?root_account_id=self
    - you should get the same results as step 3
 5. use the "List your courses" API endpoint with
    ?root_account_id=<numeric id>
    - only courses with the specified root account id
      should be returned

fixes CNVS-13522

Change-Id: I1f83dc6ac21034de4dad11f09e7f9448cf2dcd7b
Reviewed-on: https://gerrit.instructure.com/36187
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brandon Pluim <bpluim@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>