[Cashtab] Link backup wallet warning to wallet backup screen

Summary: Not intuitive, esp to a new user, where to find the wallet backup screen

Test Plan:
npm test, npm start, navigate to localhost:3000 in an incognito window so you have no storage, create a wallet, observer warning msg, click link and it goes to expected screen

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16325