Add template list preloading for users, themes and plugins

[talldan]

Description

Adds preloading for some of the requests made in the site editor template list.

This builds on #36763. I've separated it out because it wasn't easy. I think the code is complex enough to question whether it's worth doing this.

Most of the complexity comes from the fact that a REST request is required to get the data needed to preload user, theme and plugin details that are displayed in the Added By column.

Better solutions to this would be server side rendering of the page or using something like GraphQL to fetch all the required info in one go.

How has this been tested?

1. Open the site editor
2. Go to Templates or Template Parts in the navigation sidebar.
3. Data in the Added By column should load straight away (apart from images, which naturally take some time)
4. Check the network tab in the dev tools, there should be any requests for users, themes or plugins
5. Visit http://localhost:8889/wp-admin/themes.php?page=gutenberg-edit-site&postType=post (make sure you have some posts first
6. The author name should load straight away.

Types of changes

Enhancement

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• I've tested my changes with keyboard and screen readers.
• My code has proper inline documentation.
• I've included developer documentation if appropriate.
• I've updated all React Native files affected by any refactorings/renamings in this PR (please manually search all *.native.js files for terms that need renaming or removal).

[Mamaduka]

@talldan, Maybe we should start with a more minimalist approach.

• Only preload data for an active theme. Currently, the template list will only display templates for it.
• Ignore plugins for now. This feature isn't documented, and we can recommend plugins to add their preload paths.

Better solutions to this would be server side rendering of the page or using something like GraphQL to fetch all the required info in one go.

We could use Embedding for this - https://developer.wordpress.org/rest-api/using-the-rest-api/linking-and-embedding/#embedding

[talldan]

We could use Embedding for this - https://developer.wordpress.org/rest-api/using-the-rest-api/linking-and-embedding/#embedding

@noisysocks just also mentioned this to me. I wasn't aware of this feature. I think it would be a good thing to look into. I'll put this on pause and look at alternatives.

[talldan]

I had a go at adding embeds to the templates controller and it works pretty well - WordPress/wordpress-develop#2002.

Here's an example of the theme data being embeded in the response of a template:

I need to work on the Gutenberg side of this as well.

My take on it is that it's pretty firmly in the enhancement territory so I don't know about landing this for 5.9, but I'll continue chipping away at this task.