remove old conversations and conversation submission comments

fixes CNVS-12330, CNVS-9234, CNVS-8099

before you check out
- configure a user to use old conversations

test plan
- ensure that everyone get new conversations, always, even if
 they explicitly told us that they really like old conversations
 better

- as a student with an existing conversation with a teacher,
 make an assignment submission and a submission comment
- as the teacher, ensure that your unread message count did not
 increase because of the submission comment

Change-Id: If5ae7143abbc5cf5e035f5ed9ea2e5728f70cd45
Reviewed-on: https://gerrit.instructure.com/34343
Reviewed-by: Braden Anderson <banderson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>