[Cashtab] Improvements to token balance header

Summary: Define the styled component in BalanceHeaderToken.js instead of importing, as this component is used nowhere else in the app. Add padding now that this token balance appears below the standard XEC balance header.

Test Plan:
npm start, navigate to send token page, confirm it looks ok

grep -r BalanceHeaderWrap src/ and no output

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D15240