handle unpublished conversation message attachments
handle unpublished conversation message attachments

test plan:
 - send a conversation message and attach a file
 - unpublish the file (or its parent folder)
 - the recipient should see a "this file has been unpublished"
   message along with the attachment, rather than a link that
   just reloads the page

fixes CNVS-12788

Change-Id: Ia6ec329ed7935bcc01af245215ada1db170109b6
Reviewed-on: https://gerrit.instructure.com/91850
Tested-by: Jenkins
Reviewed-by: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
QA-Review: David Tan <6283e310a2278fc51bdc4fb5974f70dd9c3366b9@instructure.com>
Product-Review: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>

grahamb/canvas-lms,sfu/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,matematikk-mooc/canvas-lms,SwinburneOnline/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,HotChalk/canvas-lms,grahamb/canvas-lms,instructure/canvas-lms,venturehive/canvas-lms,matematikk-mooc/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,sfu/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,djbender/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,grahamb/canvas-lms,venturehive/canvas-lms,instructure/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,instructure/canvas-lms,matematikk-mooc/canvas-lms