Adds a drag-and-drop warning to New Files

closes CNVS-9042

Test Plan
	- Go to New Files
	- Start tabbing through until to get to the Folder Tree
	- Tab once more and you should be shown a warning of sorts
	- Tab again and you should be at the column headers

Change-Id: If2398248658e1b018610b3bfacf3294e0b3d5fa1
Reviewed-on: https://gerrit.instructure.com/45709
Reviewed-by: Dan Minkevitch <dan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Clay Diffrient <cdiffrient@instructure.com>