[Cashtab] Improve OnBoarding component

Summary: Refactor Onboarding to match preferred "index.js" and "styles.js" in ComponentName directory name. Change component so that importing a wallet opens a modal, the same UX that is currently on the Wallets screen. Add integration tests to confirm expected behavior.

Test Plan: npm test

Reviewers: #bitcoin_abc, emack

Reviewed By: #bitcoin_abc, emack

Differential Revision: https://reviews.bitcoinabc.org/D16037