A11y: Calendar headings which expand/collapse content are again accessible.

This is the other half of the fix for
efdb39da2bb3060062b703eb89dea7c5a67bd917

Fixes CNVS-16979

Test plan:
- Visit the agenda view of the calendar with a screen reader.
- Confirm that all headings that are expandable/collapsable have a control
  that can be used by screen reader users, and that it is clear that the
  control can be interacted with.
- This should also be tested in the calendar navigator section.
- Verify that keyboard only users are also able to expand and collapse these
  sections.

Change-Id: Id809c32bdbc8ba7c702083699d28b4d8520a607d
Reviewed-on: https://gerrit.instructure.com/45482
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>
QA-Review: Derek Hansen <dhansen@instructure.com>