[Cashtab] Display fiat price in local currency

Summary: Related to task T2300. XEC price in local currency displayed in balance header.

Test Plan:
cd web/cashtab
npm start
observe the XEC price in the balance header

Reviewers: bytesofman, emack, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D11178