Allow multiple users to publish to the same blog.
This enables collaborative publishing to a blog on Write.as. It requires a new concept of user permissions tied to:
- A collection owned by an organization
- User membership in the organization
- Membership permissions allowing publishing from the user
Reconfigure permission checks to run through a consistent permission system.
User-owned collections: require publishing user to be the same as the collection owner (a user).
Organization-owned collections: require publishing user to be a member of the collection owner (an organization), and have permission to publish to that collection.