Allow Challenge Stages to be taken out of order

[Charlie CM]

Aside from the QOL utility of being able to jump in to clear stages in any order, it's very frustrating when 1 player d/cs towards the end of the stage and the entire party has to replay right away to keep going, or when someone wants to help out another party but doesn't have the right class clear on the right stage.

Having to clear stages in order feels like a barrier to playing, especially given the smallish server population.