use JS for all flash notifications

this way the user experience is identical (sighted
or screenreader), whether the notification
originated from the server or the client

test plan:

1. do something that causes a flash notification
   from the server (like un/publishing a course)
   - observe the visual design animates in
   - observe that the screen reader announces
     the notification

2. do something that causes a flash notification
   from the client (like sending a message)
   - observe the visual design animates in
   - observe that the screen reader announces
     the notification

closes CNVS-12222, CNVS-12221, CNVS-12224, CNVS-14881

Change-Id: I0a61b8a12292211726bc99d23d056c92918c5f76
Reviewed-on: https://gerrit.instructure.com/39486
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>