[Cashtab] Extend Cashtab bip21 support to cover multiple addresses and amounts

Summary:
bip21 (extended for ecash) supports multiple addresses and amounts. However Cashtab does not yet support this functionality.

Implement in Cashtab according to the spec (ref: https://github.com/Bitcoin-ABC/bitcoin-abc/blob/master/doc/standards/bip21-ecash-additions.md)

Test Plan:
npm test

this diff is deployed at https://cashtab-local-dev.netlify.app/

go to the send screen and try some bip21 query strings with multiple addrs and amounts

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D16769