[Cashtab] Support Saudi Riyal

Summary: Added support for Saudi Riyal

Test Plan:
cd web/cashtab
npm start
navigate to settings tab
select Saudi Riyal as fiat currency
navigate to wallet tab
ensure Saudi Riyal is the selection and that conversion is taking place

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10618