[Cashtab] Parse eCash Chat prefixed messages

Summary: Updating Cashtab to recognize eCash Chat messages and XEC tipping transactions.

Test Plan:
npm test
npm start
Visually check inbound Cashtab, ElectrumABC and eCash chat messages (use dev link from email or ping me for a wallet with these txs)

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D15822