Up until now, the `UserActivity` class hierarchy contained things like
beatmap info, room info, full replay info, etc. While this was
convenient, it is soon going to be less so, as the data is sent over the
wire to the spectator server so that the user's activity can be
broadcast to other clients.
To counteract this without creating a second separate and slimmed-down
class hierarchy, slim down the `UserActivity` structure to contain the
bare minimum amounts of data such that the structures aren't overly
large and complex to serialise, but also contain enough data that they
can be used by receiving clients directly without having to do beatmap
or score lookups.
- make `UserActivity.InGame` and derive that to `InSoloGame` and `InMultiplayerGame`
- rename `SoloGame` to `InSoloGame`
- rename `MultiplayerGame` to `InMultiplayerGame`
