[Cashtab] Apply BigNumber to all base unit calcs in sendBch

Summary: Finalize the refactor of sendBch function away from toSatoshi methods and js math. Apply BigNumber library to remainder variable and adjust compare functions to use BigNumber methods. Add unit tests to demonstrate no change from previous function.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9180