fix date parsing on course settings page

test plan:
 1. in course settings, enter a date such as "2/11/14" in the
    term start or end date
 2. save and reload the settings page
 3. go back to editing the page and ensure the date is still
    correct (and that it hasn't been interpreted as Nov 14 2002)
 4. also ensure selecting dates from the date picker widget
    continues to work
 5. also, make sure entering something that isn't a date
    doesn't cause a page error when saving

fixes CNVS-10339

Change-Id: I8c6eb5e47757899d42af04fc24676abe86c60ec3
Reviewed-on: https://gerrit.instructure.com/28350
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>