Add extended playlist management capabilities to referee hub

[bdach]

• Continuation of Add way of realtime interfacing with multiplayer rooms from outside the game #406

Namely adding / editing / removing playlist items other than the current one.

Of particular note:

• PlaylistItemChanged event receives (via its new superclass) an Order field so that clients can know which orders the items are to be played.
• I've applied concessions in StandardMatchController wherein all referees have host-level control over the playlist (all referees can add items even if the room is host-only, and all referees can remove items that they don't own). This isn't particularly relevant yet but will be in the next changes (which will permit multiple referees in one room).
• 775d1d7 is a small reorder because in testing I've noticed that with the previous call order clients would briefly see ushort.MaxValue in the playlist order field. It is rectified by a later edit event for the same playlist item, so it wouldn't have caused a real issue, but it just felt wrong to keep. (Notably, writing this referee API and looking at how many events are dispatched, we can probably do better in reducing the number of events dispatched because some appear redundant. But that can wait for another day.)

Everything else in the diff is largely trite boilerplate.

Tested via own PoC client (bdach/osu-referee-client-poc@e1317fe).