Extend PhabricatorPolicyCodex interface to handle "interesting" policy defaults

Summary:
Fixes T13128. Ref PHI590. This is a rough-and-ready implementation of a new PhabricatorPolicyCodex->compareToDefaultPolicy() method that subclasses can override to handle special cases of policy defaults. Also implements a PolicyCodex for Phriction documents, because the default policy of a Phriction document is the policy of the root document.

I might break this change into two parts, one of which maintains the current behavior and another which implements PhrictionDocumentPolicyCodex.

Test Plan: Created some Phriction docs, fiddled with policies, observed expected colors in the header. Will test more comprehensively after review for basic reasonable-ness.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin, swisspol

Maniphest Tasks: T13128

Differential Revision: https://secure.phabricator.com/D19409