Multisite: Use WP 5.1-era hook wp_initialize_site

[kraftbj]

Fixes #11437
Requires: #12287

wpmu_new_blog is deprecated. Need to use wp_initialize_site.

Changes proposed in this Pull Request:

• Uses new hook.

Testing instructions:

• On a multisite running WP 5.1+, in Network Admin, choose option to automatically connect subsites.
• Create a new site.
• Verify no notices in the php log and the site connects.

Proposed changelog entry for your changes:

• Multisite: Use modern wp_initialize_site hook when automatically connecting new sites.

[jetpackbot]

Thank you for the great PR description!

When this PR is ready for review, please apply the [Status] Needs Review label. If you are an a11n, please have someone from your team review the code if possible. The Jetpack team will also review this PR and merge it to be included in the next Jetpack release.

Scheduled Jetpack release: May 7, 2019.
Scheduled code freeze: April 30, 2019

Generated by 🚫 dangerJS against 9fd39dd

[jeherve]

choose option to automatically connect subsites.

Did the option ever work? It is currently commented out, and I think it's always been commented out.

When I uncomment it, I can save the option, but when creating a new site it does not seem to be automatically connected to WordPress.com right now.

Maybe we could remove all of this if it's been commented out since Jetpack 2.9?

[kraftbj]

You're right; the UI is disabled. You can manually set the option (or filter) and it does work. WordCamp uses it when new camp sites are spun up.

[jeherve]

It works well in my tests. Good to merge!

Noting that I had to go through a few hoops to test with Jetpack Beta since we check for the jetpack/jetpack.php plugin on our Network admin settings pages, but not for jetpack-dev/jetpack.php.