[Cashtab] More refinements to spot price presentation in agora

Summary:
Make prices of exact integers more easily distinguishable from prices of almost integers. A price of exactly 1 XEC should be easily distinguishable from a price of 1.0035 XEC.

Update the API of getMinimumFractionDigits to not support any arbitary max. More appropriate to its use case here to take a switch of fiat or xec.

Test Plan: npm test

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17447