[Gutenberg] Prevent x-post icon showing on self-hosted sites

[SiobhyB]

Fixes WordPress/gutenberg#40546

Description

The x-post icon — + — is currently displaying in the formatting toolbar for text-based blocks on self-hosted sites. The icon is unresponsive when tapped, as x-posts currently only work with P2-based themes (which can't be self-hosted at the time of writing).

The fix proposed in this PR involves adding a check for whether a site uses the WP.com rest API before enabling the icon. This is inline with the check used to determine whether @mentions are supported on a site.

Testing Steps

To test:

• Open a post/page on a self-hosted site.
• Add a Paragraph block.
• Scroll the toolbar, located at the bottom of the screen, until reaching the last item.
• Observe that all of the formatting buttons (i.e. bold, italic, etc.) are functional and there's no longer a redundant + icon.

Screenshots

Before	After

Regression Notes

1. Potential unintended areas of impact

• The x-post icon should still be accessible from the toolbar for sites with the P2 theme enabled.
• There should be no other changes to the formatting toolbar on self-hosted sites, only the non-functional + icon should be removed.

2. What I did to test those areas of impact (or what existing automated tests I relied on)

I manually tested the existing x-post functionality on a site with P2 enabled and confirmed it worked as expected. I also tested the formatting toolbar for the paragraph block on a self-hosted site and didn't spot any unexpected regressions/changes.

3. What automated tests I added (or what prevented me from doing so)

As this was a small bug fix, I didn't determine that extra automated tests were necessary.

PR submission checklist:

• I have completed the Regression Notes.
• I have considered adding accessibility improvements for my changes.
• I have considered if this change warrants user-facing release notes and have added them to RELEASE-NOTES.txt if necessary.

[peril-wordpress-mobile]

You can trigger optional UI/connected tests for these changes by visiting CircleCI here.

[peril-wordpress-mobile]

You can test the changes on this Pull Request by downloading the APKs:

• Jetpack-pr-16507-build-123051.apk

• WordPress-pr-16507-build-123051.apk

[mchowning]

Code change makes sense and is working perfectly in my testing. Great job!

[SiobhyB]

Awesome, thanks Matt! 🎉