Provide a UI for users to modify rule-negation.

This is the UI component for fedora-infra/fmn.lib#12
> Can you really override the method that is set in the <form that easily?

Ah, yeah.  It is not actually the ``method`` of the ``form`` being overridden, it is just another bit of form data like ``openid`` or ``context``.  The actual HTTP verb being used is still POST.

> Shouldn't it be in the {% if rule.negated %} ?

Well, here I was using it like a toggle-switch.  If the rule is already negated, then if you "*negate*" it again it goes back to.. not-negated.