a11y: Gives reason for failure to create item

On the modules page this makes it so a reason is given
for why the failure occurs when it is from a blank assignment
title.

Modifies application.html.erb to have flash messages rendered
outside of #application so that they are read by screenreaders
when modals are open since everything in #application gets
aria-hidden when dialogs get opened.  Also changes
$.fn.errorBox from using '#aria_alerts' in favor of
$.fn.screenReaderFlashError. This necessitates a change in the
spec for login_logout as well.

fixes CNVS-14964

Test Plan:
  - Go to the modules page
  - Add an item
  - Choose assignment type and a new assignment
  - Finish the form without a title
  - A popup tooltip should appear saying that a blank title can't be used.

Change-Id: I7647721cf95d522d61b929ef5e948610ff67f9c8
Reviewed-on: https://gerrit.instructure.com/43799
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>