Add accessibility for Announcements RSS

fixes: CNVS-12849

Without some kind of label for this
announcements RSS link, it does not
say anything but "visited link" when
a screenreader scans it.  This adds a hidden label
so that the screenreader can say "RSS Feed".

Test Plan:
  - Login as any user with a course
  - Visit the Announcements page for a course
  - Enable your screenreader and tab through to RSS Feed
  - You should hear "link: RSS Feed"

Change-Id: I039cdd430141a28d53d04d11e6806b80eb961ebc
Reviewed-on: https://gerrit.instructure.com/34367
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Drew Bowman <dbowman@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Dana Danger <dana@instructure.com>