add screenreader text to unenroll link on user page

test plan:
* as an admin, visit the page of a user with an enrollment
* with a screenreader, the link with an "X" icon should
have descriptive text

closes #CNVS-13996

Change-Id: I2bb29fc10803028246f1fde121e5a0dba9f36452
Reviewed-on: https://gerrit.instructure.com/37932
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>