refactor calendar_event.import_from_migration

moved code into a helper class so I could add a bunch of methods
without polluting the model class. Broke out the attachment
handling into separate methods.

fixes CNVS-10126

test plan:
 - specs pass
 - migrate some calendar events and verify it still works.

Change-Id: Ia44848f03f298defb2ef009078b08fb248ffa3e8
Reviewed-on: https://gerrit.instructure.com/28226
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Reviewed-by: Zach Pendleton <zachp@instructure.com>
Product-Review: Jon Willesen <jonw@instructure.com>