Support multiple UIs bound to a single logic

I tried to keep the changes minimal. I had to add an intermediate page between creating the app instance and joining as a user to choose your UI. The page doesn't show up if only 1 UI is available (no choice to make)

I decided to add a uiId field in ClientApp, i.e. every UI of our apps need to define a unique UI identifier.

I tested the code locally. However the files on course-material need minor adjustments (simplifications, no overhead except the uiId).

Players can play on different UIs.

A drawback would be that we have a single thumbnail in the hub for all UIs of an app