Improves a11y for the content migrations page.

Adds a new flash message type for screenreader which is exclusively announced.

closes CNVS-15142

Test Plan:
	- Go to the course migrations (using a SR of course)
	- Start a migration
	- The SR should announce that a migration was queued.
	- The SR should announce progress being made on the migrations.

Change-Id: Idec9dc27ea81a3a2617670b9e255f5a37565bd57
Reviewed-on: https://gerrit.instructure.com/43933
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>