New Onboarding: redirect new users to use-your-domain flow

[razvanpapadopol]

What

Add the option to redirect to the existing use-your-domain flow on DomainStep in New onboarding for new (logged-out) users.

Why

Allow users to create wp com sites using their own domain

A/C

• [ ] Show the signup modal when a new user is clicking the button
• [ ] After successful signup/login, redirect to use-your-domain flow
• [ ] Site creation after login/signup when the flow is complete and is not AnchorFM should continue to work
• [ ] Continue to use query param when redirecting to Calypso Login and back on successful login

How

Refactor useOnLogin and useOnSignup hooks to stop creating the site by default at signup/login and do any action set by the Gutenboarding app.

Examples of such actions:

• handle site creation when flow is complete
• do nothing when the flow is AnchorFM
• redirect to use-your-domain when used in DomainStep
• start /new flow after Login / Signup

Basically we currently have createSite as the action X in the diagram below and we want it to be a configurable action.

Additional context PR that added this feature for existing users (already logged-in): https://github.com/Automattic/wp-calypso/pull/48692 Main issue: https://github.com/Automattic/wp-calypso/issues/44645

[ciampo]

Note to self on next steps, after some investigation today:

1. Refactor the hooks without changing functionality:

   • Extract logic for the createSite function out of the useOnLogin and useOnSignup hooks
   • Modify both hooks to accept a callback to execute. The callback will be a function that gets certain parameters (like userId and siteId). These parameters are going to be provided by the hooks
   • Pass the extracted createSite function as a callback to the hooks

2. Introduce the "Use a domain I own" button for logged out users

   • Show login/signup modal when clicking the button
   • Create a callback to redirect the user for the useOnLogin and useOnSignup hooks

Steps 1 and 2 should probably happen in 2 separate PR, to facilitate the review process

[razvanpapadopol]

Idea for handling Calypso Login success: pass the URL (/start/domains/use-your-domain?source=${ window.location.href }) to SignupForm and use it at https://github.com/Automattic/wp-calypso/blob/trunk/client/landing/gutenboarding/components/signup-form/index.tsx#L193