truncate new announcement notifications
truncate new announcement notifications

fixes COMMS-1923

Test Plan:
- make a new announcement
- go to /users/<id of user who has cc sms channel and has this
notification on>/messages
- notice the message content is truncated to products design

Change-Id: Ica42a07da679870218185a2879074d03b78b4888
Reviewed-on: https://gerrit.instructure.com/184644
Tested-by: Jenkins
Reviewed-by: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
QA-Review: Ryan Norton <65299b45178f94900ff304765e8fbf07bd13b331@instructure.com>
Product-Review: Steven Burnett <a0b450117c5d20a22d5c4d52bb8ae13adee003d8@instructure.com>

instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms