Shortlinks: Register as REST API post field and Gutenberg plugin

[tyxla]

In the WP.com old editor, we used to have the shortlinks handy in the sidebar. We don't have them in Gutenberg, and they can sometimes be a very handy feature. This PR registers the post shortlink as a REST API field, and as a Gutenberg plugin, which are necessary to be able to build the UI for shortlinks in Gutenberg.

Changes proposed in this Pull Request:

• Add shortlinks to the REST API post response.
• Register shortlinks as a Gutenberg plugin, so they're available in our block availability endpoint.

Testing instructions:

• Start a JN site with this branch.
• Connect the site, and activate all recommended features.
• Write a post and save it.
• Type wp.data.select( 'core/editor' ).getCurrentPost().jetpack_shortlink in your browser console.
• Verify it returns a correct shortlink that leads to the post you just created.
• Type window.Jetpack_Editor_Initial_State.available_blocks.shortlinks in your browser console.
• Verify it returns {available: true}.
• Want to test the actual Gutenberg extension that uses this? Go to Gutenberg: Add WP.com shortlinks extension wp-calypso#29475 and follow the test instructions there

Proposed changelog entry for your changes:

• Shortlinks: Register as REST API post field and Gutenberg plugin

Notes

I'm marking this as low priority, as it isn't super important to have. In the same time, I think it's handy to have it, and it's straightforward enough so we could have it ready for the next Jetpack version.

[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: January 10, 2019.
Scheduled code freeze: January 3, 2019

Generated by 🚫 dangerJS against 457833f

[jeherve]

This works well for me, except for this:

Type window.Jetpack_Editor_Initial_State.available_blocks.shortlinks in your browser console.
Verify it returns {available: true}.

For this to work, you would have to add shortlinks to the list of default plugins in class.jetpack-gutenberg.php.

I personally find this confusing to be honest, so I'll just link to my other comment about this :)
#10844 (comment)

[matticbot]

D22402-code. (newly created revision)

Addressed feedback

[tyxla]

I agree this could be confusing @jeherve! It was even more confusing because it worked for me without adding it specifically.

I've added it in 457833f, and this PR could use another review.

[jeherve]

Looking good now. 👍

[roccotripaldi]

works well!

[tyxla]

Thank you folks!

[tyxla]

Diff to manually sync this to WP.com: D22459-code