Hide < & > arrows in mini-cal from screenreaders

Modified core jqueryui datepicker :/
Updated mini calendar external

Fixes CNVS-14481

Test Plan:

I'd like to pair with who ever tests this. So I can get an idea how
these are tested.

	- Make sure the mini-cal is invisible to Screen Readers

Change-Id: Ib95c812746dabb5e5a2b25472923bb8e1e7c9ca6
Reviewed-on: https://gerrit.instructure.com/40298
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Alex Boyd <aboyd@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>
Product-Review: Brad Horrocks <bhorrocks@instructure.com>