R1:9a0dd55442d7

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…