ensure calendar events API uses correct date times part 2

fixes CNVS-9618

test plan
 - passing an invalid date or invalid
   date time to the calendar events index
 - it should return a 400 bad_request

Change-Id: Ie559a2902f07c1e13d76bb0d087372c7dfe09cf5
Reviewed-on: https://gerrit.instructure.com/33175
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Rob Orton <rob@instructure.com>