discussions: add "move to" option

fixes CNVS-9044; fixes CNVS-12110

test plan:

  * open discussions as a user with moderation permissions
  * click the gear menu on a pinned discussion topic
  * click the "Move To" option
  * select a new position for the discussion topic
  * verify that the discussion topic is moved
  * refresh the page
  * verify that the change persists

Change-Id: If118e41066d224dbb3153113c67372ecdd2b3007
Reviewed-on: https://gerrit.instructure.com/32423
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>