[Cashtab] Support one-click redemptions

Summary: Support redemptions of Firma for $1 of XEC

Test Plan:
npm test

Do it live at the test site, https://cashtab-local-dev.netlify.app/#/token/0387947fd575db4fb19a3e322f635dec37fd192b5941625b66bc4b2c3008cbf0

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17701