Collaboration improvements (collaboration v2)

Collaborators should be able to make changes within a universe just like the owner can, with the exception of adding/removing collaborators, deleting the universe, and other actions.

Owner actions:
- Everything currently supported on content, plus
- Adding users by email address as "collaborators" to a universe
- Viewing all collaborators in a universe
- Removing any collaborator from a universe

Collaborator actions (for content in a universe they've been added as a collaborator to):
- Are able to add and edit content within a universe as if they had the same subscription level as the universe owner
- e.g. a Premium user can add non-Premium users to a universe, and those non-Premium users have unrestricted access to create anything they want within that universe
- Are not able to delete content within a universe
- Are not able to modify content privacy settings within a universe

Dependent tasks:
- [ ] Send an email to users with a link to the universe when they are added as a collaborator
- [x] Keep track of who changed what on what content and display it on that content's page, #57
- [ ] If a user is invited to collaborate on a universe without an existing account, they should be sent an invitation

The issue does not name files, tests, or entry points. Start by tracing the existing owner actions on universe content and the collaborator data model; the work is done when the specified owner and collaborator permissions, email notifications, and invitations for users without accounts are implemented and verified.

Written by the indexing model from the issue text.