[Cashtab] Convert calculated fiat amount string to locale string
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Description

[Cashtab] Convert calculated fiat amount string to locale string

Summary:
Linked to T1710:

Created new formatFiatBalance function in validation.js which takes in the fiatBalance and an optional locale parameter. A locale formatted string is then returned accordingly.
Created unit tests for formatFiatBalance in validation.test.js
Updated Send.js to pass in fiatPriceString to the formatFiatBalance function
Updated UI snapshots for Send

Test Plan:

npm start
navigate to the Send UI
Ensure it is displaying zero fiat currency by default
Input 500000000 as the XEC amount and ensure the calculated fiat figure is displayed in locale format
npm run extension
Repeat above in browser extension mode

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc