#7459540 Signup: add newsletters flow

Proposed Changes

This extends the existing signup flow by adding a newsletters flow. Along with the flow I added a new function to generate the return because we will be forwarding a few other flows to stepper in the same manner.

To match the styling I added the flowName to the signup-header and removed the unnecessary div.

More details on the new flow can be found here

Testing Instructions

1. Pull branch and run yarn start
2. Navigate to http://calypso.localhost:3000/start/newsletters
3. Signup for a new account using a test email address that is NOT an Automattic address (this will end in an error if you don’t)
4. Go through the entire signup and you should be redirected to http://calypso.localhost:3000/setup?flow=newsletters&siteSlug=[SITE SLUG].wordpress.com
5. That should then put you on http://calypso.localhost:3000/setup/letsGetStarted?flow=newsletters&siteSlug=[SITE SLUG].wordpress.com

Pre-merge Checklist

• Have you written new tests for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the “[Status] String Freeze” label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?