Make new seller registration on/off

[dlresende]

Today, new sellers (participants) are able to register themselves at any point of the game. Sometimes this is not desirable, because a seller with an important negative balance can simply create a new account and start from 0.

This story is about making the registration configurable, so that the facilitator can turn it off once all teams are registered and ready to play.

[jak78]

Very good idea!