[ecash-herald] Improve formatting of xec send msgs

Summary:
Format small amounts in fiat if prices are available. Show tx fee as a fiat cost instead of 'satoshis per byte', which is really only interesting to devs.

Showing very small fiat values in fiat is more 'newsworthy' imo. Demonstrates microtransactions and provides actual information, vs satoshis per byte.

Test Plan: npm test

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Subscribers: PiRK

Differential Revision: https://reviews.bitcoinabc.org/D14187