Preview a spend rule

Where a spend rule stands now. Rolling rules have their window as period. Forecast rules have a forecast object, and their thresholds are measured against the projection. Balance rules have the burn-rate window as period, the available balance as value and a balance object. Spike rules have their window as period, the spend over it as value, the median of the same window over the previous 7 days as base, the point they fire at as next_threshold and a spike object.