[Cashtab] Replace toSatoshi method in satoshisToSend calc

Summary:
Replace the toSatoshi library method with new function toSmallestDenomination in one instances, the cacluation of the satoshisToSend amount. Add a unit test to confirm this function returns the same result as the previous method.

This is part of an ongoing refactor of the sendBch function to accept arbitrary decimal places for sending BCHA. Two more instances of toSatoshi need to be removed, but they require separate refactoring and should be in a different diff.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9127