[Cashtab] Update rendering for fees on FIRMA-USDT redeems

Summary: Fees are not a constant $2, but min $2 and 1% on amounts > $200.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien, emack

Reviewed By: #bitcoin_abc, Fabien, emack

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18203