[Cashtab] Support New Zealand Dollar

Summary: Added support for New Zealand Dollar, related to task T2061.

Test Plan:
cd web/cashtab && npm start
navigate to settings tab
select NZ Dollar as currency
navigate to wallet tab
ensure NZD is selected currency and that conversion is taking place.

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10642