update the datetime plugin to display the context time

fixes CNVS-13406

test plan:

- have a course with a different time zone than your time zone
- create/edit an assignment
- changing dates and times should show local and course time

- create/edit a quiz
- changing dates and times should show local and course time

Change-Id: Iae2cb267119fe717ebf5bf867d9cb934d18232f8
Reviewed-on: https://gerrit.instructure.com/36100
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Drew Bowman <dbowman@instructure.com>