Goals and DP first: Move DIFM intro before account

[zaguiini]

Closes #97466.

Proposed Changes

Show the DIFM intro step before redirecting the user to account creation.

Why are these changes being made?

So there's more context about DIFM before showing the user the sign up step.

Testing Instructions

1. Enter /setup/onboarding
2. Click the "Let us build a custom site for you" link

• Verify you see the DIFM intro screen and click "Get started"
• Verify you get redirected to the login screen
• Verify you enter the DIFM flow after signing in

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-128453

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-128453&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-128453&env=a8c-for-agencies

[matticbot]

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

Sections (~211 bytes added 📈 [gzipped])

name                  parsed_size           gzip_size
onboarding-flow            +506 B  (+0.5%)     +165 B  (+1.3%)
update-design-flow         +122 B  (+0.0%)      +16 B  (+0.0%)
signup                     +122 B  (+0.0%)      +16 B  (+0.0%)
link-in-bio-tld-flow       +122 B  (+0.0%)      +16 B  (+0.0%)
site-setup-wg               +97 B  (+0.0%)      +30 B  (+0.0%)
site-setup-flow             +97 B  (+0.0%)      +30 B  (+0.0%)

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 (~150 bytes added 📈 [gzipped])

name                                          parsed_size           gzip_size
async-load-signup-steps-new-or-existing-site       +184 B  (+0.3%)      +70 B  (+0.3%)
async-load-signup-steps-difm-site-picker            +68 B  (+0.0%)      +52 B  (+0.1%)
async-load-signup-steps-site-options                +43 B  (+0.1%)      +28 B  (+0.1%)

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.

[arthur791004]

I made the changes below:

• Use the back_to parameter to allow users to go back to the DIFM starting point of the onboarding flow
• Use the newOrExistingSiteChoice parameter to allow the first step of the DIFM to support pre-selected choice to go to the corresponding next step directly
• Display Use existing sites on the DIFM starting point of the onboarding flow for the logged-in users if there is no selected site.
• Fix some styles

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• notifications
• wpcom-block-editor

To test WordPress.com changes, run install-plugin.sh $pluginSlug goals-first/difm-intro-first on your sandbox.

[vykes-mac]

Working as expected, I was able to go through the DIFM flow from the Goal first screen.