[Cashtab] Allow user to receive specified amount of XEC or FIRMA on Receive page

Summary:
Cashtab send screen supports bip21 query strings. We should allow users to make their own to support point of sale use case.

This is a first pass, where we support custom XEC or FIRMA amounts.

It can be extended to support arbitary token quantities or other bip21.

new receive screen:

Test Plan:
npm test

deployed live at test site, check at https://cashtab-local-dev.netlify.app/#/receive

Reviewers: #bitcoin_abc, Fabien, emack

Reviewed By: #bitcoin_abc, Fabien, emack

Subscribers: emack, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D18061