[Cashtab] Match return type in unit conversion functions

Summary:
T2604

For artifact reasons, one conversion function returns a number while the other returns BigNumber

They should both return BigNumber

Test Plan:
npm test

npm start and test some transactions

Reviewers: #bitcoin_abc, kieran709

Reviewed By: #bitcoin_abc, kieran709

Differential Revision: https://reviews.bitcoinabc.org/D11875