a11y improvements for New Files search results

Adds a screenreader only flash message for search result counts also
splits out the no results into it's own component and rendering it alone
rather than having column headers above a text region.

fixes CNVS-16495

test plan:
	- Enable New Files
	- Activate Screenreader
	- Search for something
	- The number of results should be read to you.
	- Search for something that yields 0 results.
	- It should be announced, and you should be able to go
	  to the text and have it read using the screenreader.

Change-Id: Ifb395a9ee8d126b7dc9c0f785a00dee52b68219d
Reviewed-on: https://gerrit.instructure.com/43534
Product-Review: Ryan Shaw <ryan@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Matt Fairbourn <mfairbourn@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>