don't hard code course start_at and conclude_at times

test plan:
* should be able to set time with course start and end dates

* if setting the end date to midnight (the default if a time
is not set), confirm that a warning icon appears, explaining that
the day before the one they've set will be the last available day

fixes #CNVS-11851

Change-Id: I4403d0b9d964e49f598e0ad6b46844add4903fce
Reviewed-on: https://gerrit.instructure.com/34046
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>