[Cashtab] Support for eCash rebrand

Summary:

• Decimal shift from BCHA to XEC
• Address format change from bitcoincash: and simpleledger: to ecash: and etoken:
• Support for legacy address format
• New assets and logos
• Extension version bump with new assets and logos
• Error handling in price API; disable sending by USD if price API is unavailable or seems to be pulling legacy price

Test Plan:
npm start

• Send an XEC transaction
• Send an eToken transaction
• Send $1 of XEC

Reviewers: #bitcoin_abc, majcosta, Fabien

Reviewed By: #bitcoin_abc, majcosta, Fabien

Subscribers: majcosta, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9718