Accessibility: Labeled remove external feed link

Fixes CNVS-13902

Test plan:
- Visit the announcements page.
- Add an external feed
- Using a screen reader, insure that the remove feed link has a meaningful
  label.

Change-Id: I54797e9d150e16e3edaad0a9869aeb1d7ffdf254
Reviewed-on: https://gerrit.instructure.com/37830
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jayce Higgins <jhiggins@instructure.com>
Product-Review: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>
Tested-by: David Josse <david@instructure.com>