[ecash-herald] display amounts in fiat if prices are available

Summary:
T2972

Add new function satsToFormattedFiat that accepts satoshis and coingeckoPrices and returns a formatted string. Returns amount in XEC if dollar amount is less than 1.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14080