[Cashtab] Better display of seed phrase

Summary: Allow user to click-to-copy seed phrase. Render seed phrase in code font. Render seed phrase in rows of 4-word phrases.

Test Plan:
npm test, npm start and check out new appearance.

burned seed used in cashtab integration tests:

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D15830