fix up date formatting in datetime_field

fixes CNVS-13701

implementing the expert advice given in g/36100 comments

test plan
- ensure that course and local datetimes display properly in
 assignment and quiz edit dialogs

Change-Id: I8083fd2b3064498365039f18ec2f08cb1704eec0
Reviewed-on: https://gerrit.instructure.com/37594
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Tested-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>