- altered db schema to delete subscriptions along with topics
- extracted logic for creating and deleting topics into separate functions
- added functions to list topics and subscriptions
- delete topic and corresponding subscription if same topic is added with different token
- added more error messages and reactions
- added handling for authentication errors (subscription and topic are removed)
- inform all subscribed rooms when subscription is cancelled due to an action in another room
- added documentation for methods
- code style