ensure close button gets focused when modal opens

closes CNVS-14913

test plan:
  - enable new files
  - click the + Files button to add a file that already exists
    - the modal diaglog that displays should have the close button
      focused (i.e. you should be able to hit 'enter' and dismiss it)
  - click the item cog dropdown icon and select 'Restrict Access'
    - the modal dialog that displays should have the close button focused

Change-Id: Ifefe0103f96288e6e8fd8513f0289af12916fb02
Reviewed-on: https://gerrit.instructure.com/39664
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Jason Madsen <jmadsen@instructure.com>