add enrollment_term_id to courses api response

this is core information about a course and should be included

Test Plan:
 * Fetch a course through the api at both /courses and accounts/:id/courses
 * the enrollment_term_id should be on the course object

closes PLAT-665

Change-Id: If60e347fc40dc2c510ba999f05e058ea3bccbcc7
Reviewed-on: https://gerrit.instructure.com/41943
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>