let destroy_content_and_replace work for child attachments
let destroy_content_and_replace work for child attachments

fixes CNVS-37507

test plan
 - have a child attachment
 - call destroy_content_and_replace on it

Change-Id: I3c18d726114d7cc179de63ec9b079bd0cb9d7012
Reviewed-on: https://gerrit.instructure.com/115326
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
QA-Review: Jeremy Putnam <b030d6d6f199f844ec424882be473c2ab9adc93b@instructure.com>
Tested-by: Jenkins
Product-Review: Rob Orton <7e09c9d3e96378bf549fc283fd6e1e5b7014cc33@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,venturehive/canvas-lms