[Cashtab] Use ecash-wallet for agora partial buys and cancels

Summary: Per title. These txs are tested; the rawtxs did not change for these methods.

Test Plan: npm test, I deployed this to cashtab.io and tried some buys and sells, which worked

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D19336