Allow winrate filtering by flag

[fcalife]

e.g. on the winrate page, allow the values to be filtered by map (I assume map is a flag for most people).

[jimmydorry]

The new system now allows this, but I haven't put a UI to it yet. Devs get to queue custom queries, that are stored and presented as one-off reports.

[DoctorGester]

Not just one flag, multiple at once!