add optional section info to courses API response (fixes CNVS-12991)

Test plan:
* Hit the courses index API (/api/v1/courses)
* Note that section enrollment info is not returned
* Now, add 'sections' to the include[] parameter
* Note that section enrollment info is returned and
  is in the context of the current user. Section names,
  IDs, and dates should all be included, along with the
  type of enrollment.

Change-Id: I0c6e0c4c049a0d0596c7e66bfdd4db72aa0f51b0
Reviewed-on: https://gerrit.instructure.com/34695
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Landon Wilkins <lwilkins@instructure.com>
Product-Review: Marc LeGendre <marc@instructure.com>
QA-Review: Marc LeGendre <marc@instructure.com>