move user attachments on user merge

test plan:
* create two users
* upload files as both users
* merge one user into the other user
* confirm that the final user has the attachments
 from the merged user (after jobs have run) and that
 files with duplicate filenames are renamed
 (however, note that files that have identical content
 and names will not be duplicated)

fixes #CNVS-12264

Change-Id: Id699c24c804808737b763ec37316e03bd1f3c59f
Reviewed-on: https://gerrit.instructure.com/33769
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>