add screenreader text to unenroll link on user page
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 <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jeremy Stanley <b3f594e10a9edcf5413cf1190121d45078c62290@instructure.com>
QA-Review: Clare Strong <40d6bb3ed7fef516c73274131d5a837c16de74fd@instructure.com>
Product-Review: James Williams  <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>

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