Add per player difficulty and modifier options

[michael-r-elp]

• Adds toggles for lobby owner to enable per player difficulty/per player modifiers
• Add difficulty selector to change difficulty of selected level, even when selecting a suggested level

[michael-r-elp]

Additional changes:

• Send MpBeatmapPacket to players that joined
• Implement fallback to beatsaver when MpBeatmapPacket is missing (slightly broken)