Bug 1000323 - Force reauthentication when disabling Find My Device.

I believe this should read:
  this._checkbox.disabled = !this._disableAttempt
because if the user cancels a forceAuth, the checkbox should remain disabled. Note that right now FxA is (wrongly) firing onlogout in this case; I'm going to fix that soon: https://bugzilla.mozilla.org/show_bug.cgi?id=1023463