/disableladder now governed by `disableladder` permission

Previously, access to /disableladder and /enableladder was controlled
by the `modchat` permission, which is available to the @, &, and ~
groups by default. However, /disableladder is a very disruptive command
and it is totally unlike modchat, so it is now governed by its own
permission, which is given to the & and ~ groups by default.