UI: Add support for sats, a.k.a. "make sats the standard"

Allow the user to configure the units for displaying BTC amounts (sats, mBTC, BTC -- just like Electrum Wallet does).

This makes it more intuitive for user to deal with the decimal zeroes in a typical BTC amount.

Assuming the BTC prices tend to rise, this is a good long-term approach. People would deal in smaller and smaller amounts of BTC, which will eventually be harder and harder to represent as BTC in a "human readable" way.