cache assignment content tag info independently

test plan:
 - enable draft state in a course
 - create some assignments, quizzes, and graded discussions
   with due dates and points possible
 - add them to a module
 - the due dates and points possible should show up on
   the modules page
 - edit the due date and/or points possible on an assignment
   and return to the modules page
 - the changed information should be reflected on the modules page

fixes CNVS-11850

Change-Id: I7cb7eecc6744ff2df63888ff1ec83633e1e382a3
Reviewed-on: https://gerrit.instructure.com/32087
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mark Severson <markse@instructure.com>
Product-Review: Mark Severson <markse@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>