make module prereq more accessible

fixes CNVS-13550

-- Test Plan --

As a user that can edit module items
Given I'm using JAWS and/or voice over
When I edit a module
And I add a pre-req to the module
Then adding a pre-req should be accessible in JAWs
And the "remove" pre-req X should be a button
And the "remove" pre-req X should have a label on it

Change-Id: I421134cbf42f03bac9d7a9a9bc3d425937f7374b
Reviewed-on: https://gerrit.instructure.com/36290
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>