New features: campaign/ad unit removal; more accurate number of active campaigns

Users that use Bitcoin address to sign in can remove their objects now (this feature is meant for convenience, we don't actually remove the data from our database yet, nor we consider it to be secret).

Also we modified the algorithm that calculates amount of active campaigns. Since yesterday we don't count campaigns that didn't spend a satoshi during last 24 hours even if their balance is positive. Thus the number of active campaigns is lower and more accurate now: