refactor DiscussionTopic#change_all_read_state

fixes CNVS-10124

test plan:
  * run specs

Change-Id: I92509681a62358b25454aec948901f603893babf
Reviewed-on: https://gerrit.instructure.com/28256
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Zach Pendleton <zachp@instructure.com>