Only one thread accesses playlist and users, no need for threadsafe collection
RemoveButtons not implemented yet
Merge branch 'OneThread'

Conflicts:
	OpenPlaylistServer/OpenPlaylistServer/Endpoints/CheckInEndPoint.cs
	OpenPlaylistServer/OpenPlaylistServer/Services/Implementation/MainWindowViewModel.cs
	OpenPlaylistServer/OpenPlaylistServer/Services/Implementation/PlaylistService.cs
	OpenPlaylistServer/OpenPlaylistServer/Services/Interfaces/IMainWindowViewModel.cs