Add project maintainers instead of a single owner

[chrisdedman]

Description

Replace the assumption that every project has exactly one maintainer.

Potential model:

Project
    *
    |
    *
ProjectMaintainer
    |
    *
    |
    1
  User

Roles could eventually include owner and maintainer.

Acceptance criteria

• one canonical owner remains required
• multiple maintainers supported
• authorization rules are explicit
• public listing shows maintainers