[Cashtab] Support one-click redemptions for XECX

Summary: Users commonly have errors in setting the price to exactly 1, which is what XECX is redeemed for. Simplify this workflow.

Test Plan:
npm test

check it out at https://cashtab-local-dev.netlify.app/

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D17641