cache the module views separately per timezone

test plan:
 0. have a course with a module that will be unlocked at a
    future time
 1. enroll two teachers, and set different time zones for
    each in their user preferences
 2. each teacher should see the unlock time in her own
    time zone (not the time zone of whoever saw the page
    first)

fixes CNVS-12088

Change-Id: Ifcde9af7c7abaf2298ba59180fd0737a73945a3d
Reviewed-on: https://gerrit.instructure.com/41351
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>