Copy of Announcement to Sender, notify announcement replies

Creates a new notification preference for announcements created by
you. Now you can see what you've sent, and how it looks in specific
communication channels. This new notification preference will also
send notifications when someone responds to the announcement.

Fixes CNVS-13745
Fixes CNVS-2135

Test Plan:

- Set "Announcement Created By You" preference
- Create an Announcement
- Check /users/[:user_id]/messages for "Announcement Created By You"
- As another user comment on the announcement
- Check /users/[:user_id]/messages for "Announcement Reply"

Change-Id: Ibb16f0dc3ba34d98f749c40d1df1eb12430aecca
Reviewed-on: https://gerrit.instructure.com/40636
Reviewed-by: Joel Hough <joel@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>