honor time zone in date_shift_options

previously, date_shift_options were treated as local times even
if they had an explicit time zone. this caused start and end
times to shift since the UI provided UTC timestamps that were
interpreted as local times by the backend.

fixes CNVS-15808

test plan:
 0. set up a course with start and end times
 1. create a new empty course
 2. copy the course from step 0 into the new course from step 1
    via the "Import Content Into This Course" button, shifting
    dates
 3. confirm that the course start and end dates are correct
    (not shifted backward by the course's time zone's UTC offset)

Change-Id: Ie478966ed4a044890d3cbd4ce94957dbcbe58654
Reviewed-on: https://gerrit.instructure.com/42328
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>