Use a taxonomy instead of meta for template & template-part

[aristath]

As discussed in #24377 and a core editor meeting, we want to move away from using post-meta to store the theme, and start using a taxonomy instead.

This PR adds the taxonomy and changes all related queries to use the new wp_theme tax.

[aristath]

Terms now get properly created and assigned.
The queries seem to be working correctly.

[aristath]

On the PHP front this is OK, now working on the JS front.
Since this was imcomplete (I previously missed all the meta.theme references in the template-part JS files) I converted this PR to a draft so we can keep working on it.

[TimothyBJacobs]

If this is using a taxonomy now, can we use the automated handling we'd get for querying and response inclusion by marking it as show_in_rest?

[aristath]

If this is using a taxonomy now, can we use the automated handling we'd get for querying and response inclusion by marking it as show_in_rest?

Thank you @TimothyBJacobs! I added show_in_rest to the taxonomy now, but I'm afraid I don't quite understand what you mean by "use the automated handling we'd get for querying and response inclusion", could you please elaborate?
Right now I'm exposing wp_theme in the JSON response using this: https://github.com/WordPress/gutenberg/pull/24720/files#diff-51f3ab0134282ae12e1994208a3ab6feR261, and then I use that instead of the meta we previously had. So templatePart.meta.theme just becomes templatePart.wp_theme. Are you suggesting there's a better way?

[TimothyBJacobs]

Now that the taxonomy shows in REST, you should be able to query for it automatically using the rest_base of the taxonomy.

It should also now be automatically included in the response as the rest_base too.

[aristath]

@TimothyBJacobs
Ah OK, I see what you mean now.
The problem with the default responses however is that they use the term-ID, and what we have here is the term-slug.
So I don't think we'd be able to get what we need without an additional REST call... Instead of doing that, I simply added the term-slug in the response.
Note: after your comment I realized that there was a conflict with default responses. By default it was returning wp_theme: [ 2 ] so I renamed the slug to wp_theme_slug and it doesn't overwrite the wp_theme anymore in responses.

If you feel this can be done better then please feel free to push commits here, you are far more familiar with the REST API than I am!

[aristath]

Superseded by #27016
Closing this one since the other PR is up to date with the latest structural changes and it will be easier to work on.