calendar: support dragging to undated events

fixes CNVS-10796

test plan:
  * open calendar2
  * open the undated events section
  * drag a dated event to the undated section
  * verify that the event becomes undated
  * verify that this functionality works with variable due date
      assignments, scheduler items, and per-section events

Change-Id: Ifd5b46d2c6480b66ac4e1268ae7694c77dc17717
Reviewed-on: https://gerrit.instructure.com/33272
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>