Inline custom policy rules inside policy capability explanation dialogs

Summary: Ref T13411. When users click a link to explain a capability (like the policy header on many objects, or the link next to specific capabilities in "Applications", "Diffusion", etc), inline the full ruleset for the custom policy into the dialog if the object has a custom policy.

Test Plan: {F6856365}

Maniphest Tasks: T13411

Differential Revision: https://secure.phabricator.com/D20805