Add checks for valid mod combinations to give pp

[smoogipoo]

• Depends on Move performance processing into ApplyToUserStats #8
• Allows all basic mods which osu!stable allows, taking note of particular edge cases like ManiaModHardRock.
• Allows classic mod (stable-like gameplay).
• Allows muted mod (doesn't change gameplay, could be useful for streamers, doesn't affect difficulty).
• Allows daycore mod (equivalent to half time for difficulty purposes).
• Disallows everything else (0 pp).