make screenreader ux in dialogs better

- removed `role="dialog"` because that puts the
  content into "application" mode, royally
  messing up navigability

- added `aria-hidden="true"` to all of the content
  outside of the dialog

- rearranged some opening/closing tags in app
  layouts so they live in the same file

closes CNVS-14476

Change-Id: I83be208a2f993caf8b436112da1b3e30e0f533fa
Reviewed-on: https://gerrit.instructure.com/39933
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Florence <ryanf@instructure.com>