Improve control over groups

[Steell]

This is a rather vague title.

The restriction is that group names have to be unique, but sometimes it would make sense for group names to be the same.

Solution 1: group_display option, overrides display setting for groups. This allows multiple groups to have the same name visually, when under the hood they are still distinct.

This would solve the micazoid problem by allowing the RTV/Nomination definition to contain all the groups of the Random Mapcycle definition, but will still display "Standard Maps" in the tiered vote due to the group_display setting.

Solution 2: More group overrides.

Solution 3: Move previous-played exclusion to its own module.