[Cashtab] Support UAE Dirham currency

Summary: Implement support for AED fiat. Coingecko supports so this is a straightforward add.

Test Plan: npm test, npm start and switch to the currency in settings, check that amount is in line with FOREX rate

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Subscribers: emack

Differential Revision: https://reviews.bitcoinabc.org/D14528