Support multiple Round Robin host groups, selecting one host per group

Support multiple Round Robin host groups, selecting one host per group: a task in bird-critic-1.5-full-300 (Harbor dataset). Round Robin event types currently draw a single lucky user from one shared pool. This PR lets organizers partition non-fixed hosts into named host groups ; the Round Robin…

The task

Round Robin event types currently draw a single lucky user from one shared pool. This PR lets organizers partition non-fixed hosts into named **host groups**; the Round Robin algorithm selects one host from each group, so every group is represented in every booking. A host belongs to at most one group.

Part of kochsnow/bird-critic-1.5-full-300.