[Cashtab] Fixate header

Summary: As per feedback from D15188, this diff fixates the header in order to better facilitate tx history pagination in D15188.

Test Plan:
npm test
npm start
click on a few txs to expand their contains and take up extra vertical space, scroll up and down and ensure the header and footers are fixed
npm run build, or load d15277.netlify.app on ios and android mobile devices and ensure header is fixed while scrolling history
npm run extension and ensure the same fixed header behavior

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D15277