[Cashtab] Use ecash-lib to sign and verify msgs

Summary: Now that we have the methods in our own stack to do this, implement in Cashtab. This allows us to stop using bitcoinjs-message.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17778