R1:2cfc3acf3234

Allow Herald pre-commit rules to act on repository projects

Summary:
Fixes T4264. Adds:

- New "Repository's projects" field to Herald pre-commit rules, so you can write global rules which act based on projects.
- Allows pre-ref/pre-content rules to bind to projects, and fire for all repositories in that project, so users with limited power can write rules which apply to many repositories.
- The pre-ref and pre-content classes were starting to share a fair amount of code, so I made them both extend…