[Cashtab] Add support for Nigerian Naira

Summary: Related to T2780. Added support for Nigerian Naira.

Test Plan:
cd web/cashtab && npm start
navigate to settings page
expand the currency drop down
select Nigerian Naira as the default fiar currency
observe that the conversion rate matches the conversion rate found here:
https://www.coingecko.com/en/coins/ecash/ngn#:~:text=1%20XEC%20%3D%20%E2%82%A60.013349372254,Last%20updated%2003%3A29AM%20UTC

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D12519