[Cashtab] Move settings validation and supported fiat currency list out of Ticker.js and to settings

Summary:
T3261

It makes sense for cashtabSettingsValidation to live with cashtabSettings in config/cashtabSettings.js

Also makes since for list of supported fiat currencies to live here, as this is a settings option.

Test Plan:
npm test
grep -r fiatCurrencies src/ and no output
grep -r settingsValidation src/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D14366