Show current policy on config form

Problem/Motivation

Currently, viewing the computed policy requires saving the admin form and inspecting the headers of the response.
It would be helpful to provide a preview of the policy parsed from the form inputs prior to saving.
The preview should include a disclaimer about (1) dynamic changes and (2) empty directives not being included.
The preview does not need to include the optimizations to reduce header length that the PHP class performs.

Remaining tasks

User interface changes

A new info box on the config form

API changes

None

Data model changes

None