[Cashtab] Moving BalanceHeader to stand alone component

Summary: Per T1691. The BalanceHeader component is re-used in 4 screens. This diff puts the code logic in one file so that changes to the component do not need to be repeated across multiple files.

Test Plan: npm start and confirm balance display matches the one at cashtabapp.com on the Tokens.js, Send.js, Wallet.js, SendToken.js screens.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D9860