[Cashtab] Deprecate ecashjs-lib

Summary:
T2893

This diff deprecates ecashjs-lib from Cashtab and replaces it with BitGo's utxo-lib. Unfortunately, we are still waiting for Bitgo to publish a version of utxo-lib that includes ecash transaction support. However, it is probably worth doing this drop-in now with the shims (i.e. the 0x01 | 0x40 in cashMethods.js) and then upgrading it later.

Test Plan:
npm test

npm start
send an XEC tx
send an XEC tx with a cashtab msg
send an XEC tx with an encrypted cashtab msg
send an etoken tx
sign a msg
verify the msg

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D13679