fix checkbox and radio button labels

fixes CNVS-12202

test plan:
  * open the edit page for a discussion topic in Firefox
  * click the label "Allow threaded replies"
  * verify that its checkbox is toggled
  * regression test checkboxes and radio buttons

Change-Id: I7bea6f82307b3834ba7f69de3d3c215386f26e26
Reviewed-on: https://gerrit.instructure.com/32847
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>