Convert BitcoinAmountField to use Amount class
Summary: Depends on D747
Test Plan:
- make check
- run src/qt/bitcoin-qt --testnet and try sending and receiving some transactions.
Reviewers: deadalnix, freetrader, mbhinder, #bitcoin_abc
Reviewed By: deadalnix, #bitcoin_abc
Differential Revision: https://reviews.bitcoinabc.org/D748