Accessibility: Names of external links are no longer obscured for SR users

Fixes CNVS-14483

Test plan:
- Add a link to an external site, such as Google, to a wiki page
- View the page with a screen reader and verify that the name of the external
  link is read, and that the text informing the user that it is an external
  link is appended to the link name.
- Verify that the visual appearance and opporation of the external link is
  uneffected

Change-Id: I689045fa09191239637dc686352bf6a0728ead0e
Reviewed-on: https://gerrit.instructure.com/38499
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Product-Review: Aaron Cannon <acannon@instructure.com>