Add support for disabling a policy

Problem/Motivation

While working on new policies (especially while migrating from swift mailer) I would like to be able to disable a policy temporarily, so that I can test another configuration without having to remove an existing policy completely. This would allow us to test new changes without having to touch the original policy. If a rollback is needed we could simply remove the new policy and activate the old one again, instead of trying to remember what the settings were for it.

Proposed resolution

Add a checkbox for disabling a policy from the policy editor or a toggle button in the action button dropdown of the policy.