fix disabled checkboxes in handlebars helper

fixes CNVS-12205

We shouldn't send the unchecked value from the hidden input when
the checkbox is disabled.

Change-Id: Ie8763c2495f17f6fc0f0f26c0679ed9f6a2b1c0d
Reviewed-on: https://gerrit.instructure.com/32827
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
QA-Review: Cameron Matheson <cameron@instructure.com>