Add a/b test for new user segment step

[fditrapani]

This PR creates an A/B test for our new site segment screen outlined here: p5XAZ9-1Cn-p2.

Testing new user

variant

• Visit /start either logged out or in an incognito window
• Ensure a/b test signupSiteSegmentStep is set to variant
• You should see the new sign up screen pictured above with the last experience question.

control

• Visit /start either logged out or in an incognito window
• Ensure a/b test signupSiteSegmentStep is set to control
• You should see this existing signup screen.

Testing existing user

• Visit /start logged in to an existing account
• Ensure a/b test signupSiteSegmentStep is set to variant
• You should see the new sign up screen pictured above without the last experience question.

@markryall and @taggon can you take a look?

[matticbot]

Test live: https://calypso.live/?branch=add/site-segment-ab-test

[markryall]

Just one small recommendation regarding the designType being provided as a dependency from the about step.

[markryall]

I think you might also need to add designType to the call to SignupActions.submitSignupStep in the about step component.

It's pretty confusing that we pass designType around in two different ways - something we should cleanup later across all flows I think.

[fditrapani]

Good catch. Thanks @markryall. I've updated it and tested it out. Looks good except for a couple errors being thrown at the end of signup. Not sure if it's related to my code or not. Can you take a look again?

[markryall]

I didn't see any errors - just a warning from 'client/signup/processing-screen/index.jsx' about assigning directly to state instead of calling setState (which wasn't anything that your PR touches).

Looks good 👍