Accessibility: Labeled the attachment field for discussions

Fixes CNVS-13729

Test plan:
- Reply to a discussion.
- Using a screen reader, add an attachment, and verify that the
  attachment field is labeled.

Change-Id: I193b18a210e3255deb61f27f50349d74f395f6e8
Reviewed-on: https://gerrit.instructure.com/36863
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Braden Anderson <braden@instructure.com>
QA-Review: David Josse <david@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>