A new source of exchange rates data and a private API endpoint

[wallacewest]

Description and Actual behavior

The app currently uses a public API endpoint (blockchain.info) to fetch exchange rates, leading to smaller amount of fiat currencies available and no protection against outages.

Expected behavior

Using a cached proxy endpoint for another, maybe a paid, SLA offering (https://currencylayer.com/ looks promising) provider, thus having a greater control over the data served and larger number of currencies to choose from.