A11y: Add content to Module dialog now resets focus after close

Fixes CNVS-15552

Test plan:
- Open modules page.
- Click the button to add content to a module.
- Close the modal that appears.
- Verify that the focus returns to the triggering control that was used to
  open the dialog.

Change-Id: Ibbf93a4fdf6e3eeb56e0e2dcf15881059e247d28
Reviewed-on: https://gerrit.instructure.com/41308
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>