Add command for applying mod multiplier changes

[bdach]

Prerequisite for ppy/osu#27685

[bdach]

Just reminder to self / anyone else that may be reading that ppy/osu#30506 is going to need special handling here because it was deployed straight-to-live without any of the relevant considerations in place and care will need to be taken that those scores don't get mangled by any multiplier recalcs.