extract attachment importer

fixes CNVS-12245

test plan:
*run a course migration on a course that has attachments
the attachments should come across

Change-Id: I897ec39bfa393835ee20cb8ff83918dcdddf57ff
Reviewed-on: https://gerrit.instructure.com/32965
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>