Signup: add newsletters flow

[arcangelini]

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)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-38723

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-38723&env=jetpack

[matticbot]

Here is how your PR affects size of JS and CSS bundles shipped to the user's browser:

Sections (~83 bytes added 📈 [gzipped])

name             parsed_size           gzip_size
signup                +466 B  (+0.1%)      +83 B  (+0.1%)
jetpack-connect       +438 B  (+0.0%)      +77 B  (+0.0%)
accept-invite         +438 B  (+0.1%)      +77 B  (+0.1%)

Sections contain code specific for a given set of routes. Is downloaded and parsed only when a particular route is navigated to.

Async-loaded Components (~56 bytes added 📈 [gzipped])

name                             parsed_size           gzip_size
async-load-signup-steps-plans          +45 B  (+0.0%)      +18 B  (+0.0%)
async-load-signup-steps-emails         +45 B  (+0.0%)      +19 B  (+0.1%)
async-load-signup-steps-domains        +45 B  (+0.0%)      +19 B  (+0.0%)

React components that are loaded lazily, when a certain part of UI is displayed for the first time.

Legend

What is parsed and gzip size?

Parsed Size: Uncompressed size of the JS and CSS files. This much code needs to be parsed and stored in memory.
Gzip Size: Compressed size of the JS and CSS files. This much data needs to be downloaded over network.

Generated by performance advisor bot at iscalypsofastyet.com.

[alshakero]

Love this. Pretty neat and satisfying. We might need some customization soon. But for now, this can be shipped.

[a8ci18n]

This Pull Request is now available for translation here: https://translate.wordpress.com/deliverables/7459540

Hi @arcangelini, could you please edit the description of this PR and add a screenshot for our translators? Ideally it'd include this string: Newsletters

Thank you in advance!

[a8ci18n]

Translation for this Pull Request has now been finished.