Don't allow squad balances to go negative

Before this change it was possible for squad balances to occasionally go negative, if funds were withdrawn and then a future game was subsequently cancelled or players were marked out (both of which triggering a refund). This created an awkward situation where squad owners would owe Squaddle money.

To prevent this, we've introduced the concept of "available balance" and added new checks to ensure that withdrawals can only be made when this is high enough. We've also prevented cancelling games (and triggering refunds) if this would push the balance negative.