fix calendar event creation creating undated events

test plan:
 - preconfiguration variants:
   - with & without user set to a different timezone
   - with & without different locales (e.g. Japanese)
 * navigate to the calendar
 * click on a day to begin creating an event
 * fill in the date and start/end times
   - with & without valid start/end times
 * click 'More Options'
   - ensure the edit page includes start/end times as typed
   - correct invalid start/end times, if necessary
 * click 'Update Event'
   - ensure the event appears on the calendar with the correct date/time
 * click on a day to begin creating another event
 * fill in the date and start/end times
 * click 'Submit'
   - ensure the event appears on the calendar with the correct date/time

fixes CNVS-11067

Change-Id: I31f6b1d7550b166a90c45d3f02132379c6dd7459
Reviewed-on: https://gerrit.instructure.com/40374
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>