Accessibility: Added label to criterion long description in rubrics

Fixes CNVS-13129

Test plan:
- Visit the rubrics page.
- Click add rubric.
- Click to show the long description.
- With a screen reader, verify that the long description field is labeled.
- Edit an existing rubric, and verify that the same field is also labeled.

Change-Id: Iccbda8a4b29676345e70b69bf5fee142a5d71aed
Reviewed-on: https://gerrit.instructure.com/36759
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>