Add guest to user role enum

[apophisnow]

This change supports the party mode feature by allowing for a guest type.

[marcelveldt]

Thanks, @apophisnow

[marcelveldt]

In hindsight we should have gone with an IntEnum for the UserRole so you can do things like require user role >= user etc
Maybe we can still do that but it will be a breaking change massively. So maybe instead we can add some mapping into the UserRole to an int value or make it a IntEnum but override its str method ?