api modifications for rollcall

refs CNVS-3451

test plan:
  - make api call to get users in a course
    "/api/v1/courses/#{course_id}/users"
    with enrollment_state[]=active&enrollment_state[]=invited&
    enrollment_state[]=completed
  - all users who were invited to the course
    are returned even if they arent in it anymore
  - make api call to get sections
    "/api/v1/courses/#{course_id}/sections"
    with "enrollments" in the included section
  - all user objects have an attached enrollment

Change-Id: I24d6ee31ca9a5c044efa526e762aa1d504045b99
Reviewed-on: https://gerrit.instructure.com/44717
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>