Feat: Subscriptions api

[bpolaszek]

WIP

• [x] Implement Subscription model
• [x] Expose methods to store/retrieve subscriptions in storage
• [x] Expose methods to dispatch new / stale subscriptions in transport
• [x] Dispatch subscriptions in subscribe controller
• [x] Expose an endpoint to show current subscriptions
• [ ] Expose the subscription item endpoint
• [x] Restrict this endpoint to authorized visitors (with topic selectors)
• [x] Expose filters as defined in the specification
• [x] Create unit tests
• [x] Create integration tests
• [x] Fix topic/subscriber order
• [x] Retrieve last event ID