A11y: Labeled Remove Attachment button on Edit Discussion screen
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Brad Horrocks <7b5c5d1f2eba6c540b8e53f17d87a9bd7dbf604b@instructure.com>
QA-Review: Steven Shepherd <e794458448443812fb2685675aab3bbd1a3891bc@instructure.com>
Product-Review: Matt Fairbourn <5f353e1c97bb9cc046b9db20beca0b9392f6f1b2@instructure.com>

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