update timestamp on discussions when merging users

fixes CNVS-12782

discussions cache some material in the "materialized view". that
material may include references to the use being deleted in a user
merge. the source material was already being updated correctly by the
merge, but the materialized view was not expiring. updating the
updated_at on the appropriate topics will force the necessary
materialized views to be refreshed.

test-plan:
 - create two distinct users, A and B
 - as user A, create a discussion topic
 - as user B, create a second discussion topic
 - as user A, reply to user B's discussion topic
 - merge user A into user B
 - view the first discussion topic; user B should display as the author
 - view the second discussion topic; user B should display as the
   reply's author

Change-Id: I48948e171b6645a371e92c8413fa289695d3d1a2
Reviewed-on: https://gerrit.instructure.com/34445
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Jacob Fugal <jacob@instructure.com>