CC author on conversation create

Now it is possible to set a notification preference to receive
notifications when you create a conversation.

Fixes: CNVS-13744

Test plan:

Set conversation created notification preference
go to inbox
create a new message and send it
go to /messages to verify there is a notification for the new conversation

note: you will have to have a verified channel to see the new message

Change-Id: I662f2eb62806183871adcce39602d639dbbfc0a3
Reviewed-on: https://gerrit.instructure.com/40873
Reviewed-by: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>