Add workflow integration with example workflow

Adds:

- Example publisher workflow with lawgiver
- Workflow configuration machinery, telling the publisher what each state / transition means
- Constraint definition machinery, with context state helper
- Transition & delete subscribers, adding publisher jobs when the workflow is a publisher workflow
- `publisher-modify-status` view, applying the constraints for a transition and redirecting to the default transition action

@maethu could you take a look at it? sorry :wink: