move migration importers to app/models/importers

closes CNVS-12404

test plan:
  - verify that importing calendar event data continues to work
  - verify that importing with media tracks continues to work
  - verify that importing discussion topics continues to work

Change-Id: Ia5c88cc179e3cc8075270d0d423e618f80d61234
Reviewed-on: https://gerrit.instructure.com/33314
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>