Publish a Blog: Add feature flag and pass it to site creation endpoint

[zaguiini]

Closes #99453.

Testing Instructions

Checkout 172681-ghe-Automattic/wpcom in your sandbox and proxy yourself.

1. Ensure you're in the cumulative cohort of 22180-explat-experiment
2. Enable the onboarding/enable-write-goal-features feature flag and go to /setup/onboarding
3. Pick the "Publish a Blog" goal (which will set the write intent on the site)
4. Create the site (you can leave after the processing screen, don't need to go through the Launchpad)
5. Enter /settings/newsletter/%s and verify that the "Add the Subscribe Block at the end of each post." toggle is ON.

Create a site in a different flow or by picking different goals and verify that the the toggle is OFF in the Newsletter settings.

[matticbot]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133091

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133091&env=jetpack

Automattic for Agencies live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-133091&env=a8c-for-agencies

[matticbot]

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

Sections (~1119 bytes added 📈 [gzipped])

name                     parsed_size           gzip_size
import-hosted-site-flow      +1139 B  (+0.1%)     +375 B  (+0.1%)
copy-site-flow               +1139 B  (+0.1%)     +499 B  (+0.2%)
link-in-bio-tld-flow          +289 B  (+0.0%)     +129 B  (+0.0%)
write-flow                     +47 B  (+0.0%)      +25 B  (+0.0%)
update-design-flow             +47 B  (+0.0%)      +21 B  (+0.0%)
site-settings                  +47 B  (+0.0%)      +23 B  (+0.0%)
settings                       +47 B  (+0.0%)      +23 B  (+0.0%)
hundred-year-plan              +47 B  (+0.0%)      +23 B  (+0.0%)
hundred-year-domain            +47 B  (+0.0%)      +24 B  (+0.0%)
hosting                        +47 B  (+0.0%)      +23 B  (+0.0%)
home                           +47 B  (+0.0%)      +25 B  (+0.0%)
build-flow                     +47 B  (+0.0%)      +25 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.

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.

[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 onboarding/write-features-ff on your sandbox.

[p-jackson]

Should we also do a treatment_cumulative check here?

[zaguiini]

I'm wondering if we should include the treatment_cumulative checks right now. It's under a feature flag, so it will only work for explicitly-enabled people. I wonder if we should only check for the experiment when it's time to remove the flag.

[p-jackson]

🤷 Either options's fine. Putting it in now means at least we know we've remembered to add that logic.

[p-jackson]

I say we leave the isGoalsFirstCumulativeExperience check in there.