rearrange enrollments API query

avoid inline includes, cause they kill performance in very large courses and it's
trying to sort huge amounts of data

test plan:
 * regression test enrollments list API (from a course, a section, and a user)

Change-Id: Ie5bd4085e3520f918c1c86b611f236c751b5474f
Reviewed-on: https://gerrit.instructure.com/27553
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>