[Cashtab] Remove mint artifact object of arrays style input

Summary: Limit the sendBch function to one address and one amount in units of BCHA. Refactor Send.js and tests to reflect new input style.

Test Plan: npm test

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9161