[Cashtab] Make the header balance a localeString

Summary:

• Updated formatBalance in @utils/CashMethods.js which is used to format the header XEC balance. The new function takes in the balance and an optional locale parameter and returns a formatted value accordingly. This function previously hard coded spaces for large balances.
• New unit tests created in @utils/__tests__cashMEthods.test.js for the redesigned formatBalance function.
• New snapshots generated across Wallet, Send and Tokens UI.

Test Plan:

• npm start
• navigate to Wallet, Tokens and Send UIs and ensure the header XEC balance is displayed correctly per locale.
• npm run extension
• Check header XEC balance is displayed per locale in browser extension mode

Reviewers: bytesofman, #bitcoin_abc

Reviewed By: bytesofman, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D10319