hide drop options for ineligible assignment groups

fixes CNVS-11852

Test plan:
  * start to make a new assignment group, the dialog should not have
    any options ignoring scores
  * edit an existing assignment group (that has assignments), you
    should see the usual drop options

Change-Id: I5293be9cea81044c0db08eb7593068647157cd5a
Reviewed-on: https://gerrit.instructure.com/32929
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>