Accessibility: Made popovers work better for keyboard and screen reader users

Popovers now close when tabbed away from, and they don't use the dialog role.

Fixes CNVS-14732

Test plan:
- Using the keyboard only
  - Visit the Calendar Agenda view.
  - Create an event.
  - Tab to and press enter on an event to open the popover for that event
  - Confirm that focus is moved to the first item in the popover
  - Tab out of the popover and verify that it closes automatically.
  - Verify that shift-tabbing out of the popover has the same effect.
- Using a screen reader
  - Verify that the popover opens.
  - Verify that the user is taken to the dialog upon open.

Change-Id: I2ca91c8781be585c689a49ef5a132cf9e509df4a
Reviewed-on: https://gerrit.instructure.com/40576
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>