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.

These rules only show up where they make sense. For example, the "Task Author" rule is only available in Maniphest, and in "Default View Policy" / "Default Edit Policy" of the Application config.

This should make T8488 easier by letting us set the default policies to "Members of Thread", without having to create a dedicated custom policy for every thread.

• Set tasks to "Task Author" policy.
• Tried to view them as other users.
• Viewed transaction change strings.
• Viewed policy errors.
• Set them as default policies.
• Verified they don't leak into other policy controls.