allow setting start_at, end_at for enrollments api

fixes SIS-160

Test Plan
---------
  - Create an enrollment via the api with start_at and end_at parameters
  - verify start_at and end_at are poplutated for resulting record in enrollments table
  - verify user can only access course during start_at and end_at times

Change-Id: Icfeaded1f43c6b0309d41280d7caa17482b9182e
Reviewed-on: https://gerrit.instructure.com/30136
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ken Romney <kromney@instructure.com>
Reviewed-by: Duane Johnson <duane@instructure.com>
Product-Review: Duane Johnson <duane@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>