Add a service to manage the basic room topic subscription for users and add an HTTP endpoint
The ws gateway counts users in a room based on a STOMP subscription to topic /topic/<roomId>.stream.
Every user can only subscribe to one basic room topic.
This subscription will be tracked by STOMP's sub-<id>.
Add a GET Mapping within a new controller to expose user count.
New endpoint: /roomsubscription/usercount?ids=<UUIDs>
Returns List of Int.
In relation to: https://gitlab.com/arsnova/arsnova-webclient/-/merge_requests/686
Edited  by Tom Käsler