A11y: Labeled Remove Attachment button on Edit Discussion screen

Fixes CNVS-16550

Test plan:
- Create a discussion with an attachment.
- Edit that discussion.
- Using a screen reader, verify that the remove attachment button is labeled
  and usable.
- Using a screen reader, further verify that the button to add another
  attachment is accessible.
- Using the keyboard only (no screen reader) reverify the above.

Change-Id: Ic6a3b3b7fd9178892a0885e807a68bf6afcebfa4
Reviewed-on: https://gerrit.instructure.com/44019
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Brad Horrocks <bhorrocks@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>