Add support for "freestyle" in multiplayer

[smoogipoo]

This is an MVP. @ppy/team-web please take this over.

I'm ferrying this via the "beatmapset_id" field which, when set, indicates that players in the room are able to individually pick their own choice of beatmap and ruleset.

I have added what I believe is one important comment where the osu!web team will need to add validation as required. Importantly:

• Users should not be able to pick beatmaps outside of the current set.
• Users should not be able to pick beatmaps with lengths exceeding +/- 30s from the base playlist item beatmap ($playlistItem->beatmap_id).
• When "freestyle" mode is enabled, required_mods/allowed_mods should be kept empty.

Feature request: The score controller (rooms/{roomId}/playlist/{playlistItemId}/scores/{scoreId}) should return a beatmap model for each score rather than just a beatmap_id.