fix date shifting for re-imported items

(also keep track of other imported items as well)

test plan:
* import a canvas export package (such as the one
referenced in the ticket) with assignments and graded
discussions
* import them into a course
* delete the assignments, etc. from the course
* re-import the package, while shifting the dates
* the re-imported (i.e. undeleted) assignments should
 have their dates shifted

fixes #CNVS-13252

Change-Id: I396059cdcf58fe3386f21cc7cffe62ac8951cd05
Reviewed-on: https://gerrit.instructure.com/36743
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>