Fixes post-grade date picker to not stick on last change

Fixes: SIS-383

Modifies the date picker to check onClose instead
of on change, so that no update is made to the
errors count until the calendar is no longer up.

Test-Plan:
--With Post to SIS feature flag turned on--
1.) Make at least 1 assignment with no due date
2.) Navigate to grades
3.) Click post to sis
4.) Verify that you can update the due-date without
	it locking the calendar on the screen

Change-Id: I7ca143343d65393e5012a62f5b757b1caf47d936
Reviewed-on: https://gerrit.instructure.com/43886
Reviewed-by: Nick Houle <nhoule@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>