A11y: Assignment due dates are now read by screen readers

Fixes CNVS-16169

Test plan:
- Create assignments with and without due, start, and end dates.
- Using a screen reader, on the show assignment page, verify that such dates
  are visible to screen reader users, and all of the  information contained
  in the tooltip that shows up on hover is also read.
- Verify that appropriate text is read when no date is set.
- Verify that the visual appearance remains unaffected.

Change-Id: I0daaf64945673f5187602454a122ddf79290ad6c
Reviewed-on: https://gerrit.instructure.com/44030
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>