[Cashtab] Patch tx parsing for agora buy/sell/cancel

Summary: We can't assume the p2sh input is at index 0. Add a testcase for an agora partial tx and modify parsing to check all inputs.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16994