send all available balance button

This adds a simple way to send all balance in the UI. 
fixes https://github.com/bitpay/copay/issues/693
That's the logic we've been using in the whole app so far. I agree in that it's too naive, but for now it's OK. Maybe we can merge this and open a new issue to improve the fee calculation for UI? I'm going to go ahead and do that, if you agree please merge this PR: https://github.com/bitpay/copay/issues/735