[Cashtab] Add support for HKD

Summary: Added support for Hong Kong Dollar. Related to task T2013

Test Plan:
cd web/cashtab
npm start
navigate to settings tab
set currency to HKD
navigate back to wallet tab
ensure currency is set to HKD and that conversion is taking place

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10595