Queue visitors until manual releasing

[jakmeier]

When visitors finished the time traveling between origin and destination, they should no longer visit instantly. Instead, they should be queued until the player releases them in the dock.

• [x] Game Master changes in attack system
• [x] GraphQL API for reading number of queued visitors
• [x] Rest API for releasing
• [x] Dock view for showing visitors and releasing them

Related issues

29 is a prerequisite

24 describes parts of the motivation and might be resolved with the implementation of this

[jakmeier]

1a7af7dffaf has the functionality complete (UI could be improved, though)