Lay most groundwork for Herald object rules

Summary:
Ref T4264. This gets most of the plumbing in for "object" rules, which will bind to a specific object, like a repository or project.

It does not yet let you actually create these rules.

Test Plan: Ran `storage upgrade`, created/edited rules, browsed Herald.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4264

Differential Revision: https://secure.phabricator.com/D7847