R1:8eed5b1f1449

Make HeraldRule implement PhabricatorPolicyInterface

Summary:
Ref T603. Ref T2769. Herald currently interacts with policies in a bad way; specifically, I can create a rule which emails me for everything, and thus learn about objects I can't otherwise see.

This shouldn't be possible, so I'm going to reduce personal rules to have only the viewer's scope.

For global rules, I think I'm always going to let any user edit them, but make who the rule acts as part of the configuration. There will be an option to…