Possible lack of validation of invalid mod combos

[0xallie]

I modified osu!lazer to allow any mod combos to see what happens. I only tested this on dev.ppy.sh for obvious reasons, which doesn't seem to actually rank any scores, but the fact that this shows up warrants a check to make sure the prod server isn't accepting such scores.

[peppy]

This will be covered by anti-cheat external to osu-web. Adding checks in osu-web could be considered optional.

Will leave it up to the team's discretion whether to consider or close.

[cl8n]

there is already code to validate mod combos but some of it (assertValidExclusivity) is only used for multiplayer currently. at a glance I'm not sure why solo and multiplayer differ here

[nanaya]

it's mainly for room validation, not score