Accessibility: Labeled the buttons for editing and deleting rubrics

Fixes CNVS-13127

Test plan:
- Using a screen reader, visit the Rubrics page.
- Verify that the edit and delete buttons are properly labeled and that those
  labels are read correctly by screen readers.
- Verify that screen reader users are able to successfully activate those
  controls.

Change-Id: Ic033b43f2ed296f63acdc3476973069d663d3ca4
Reviewed-on: https://gerrit.instructure.com/36697
Reviewed-by: Cameron Matheson <cameron@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>