[Cashtab] Patch displayed spot prices for agora offers

Summary:
We need to display the spot price in XEC per decimalized token, not XEC per token satoshi.

Right now the displayed prices are only right for 0-decimal tokens.

Test Plan: npm test

Reviewers: #bitcoin_abc, tobias_ruck

Reviewed By: #bitcoin_abc, tobias_ruck

Differential Revision: https://reviews.bitcoinabc.org/D16991