Distinguish between parties that allow drop-in and those that don't

A continuation of https://paliaparty.featurebase.app/p/flexible-party-durations

This will allow parties to support "drop-in, drop-out" play by keeping parties active and signal to players that they may enter after the party has started. If players need to leave early, the party will remain visible, allowing new players to jump in and fill empty slots mid-party.