[Cashtab] Added support for AUD

Summary: Added support for Australian Dollar, related to task T2060. Resubmitting due to rebase error.

Test Plan:
cd web/cashtab && npm start
navigate to settings tab
select AUD from dropdown
navigate to wallet tab
ensure AUD is the selected currency & that conversion is taking place

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10627