R1:466755476aed

Allow PolicyRules to serve as "Object Policies"

Summary:
Ref T5681. Ref T8488. This allows policy rules to provide "Object Policies", which are similar to the global/basic policies:

- They show up directly in the dropdown (you don't have to create a custom rule).
- They don't need to create or load anything in the database.

To implement one, you just add a couple methods on an existing PolicyRule that let Phabricator know it can work as an object policy rule.

{F494764}

These rules only show up…