Use the image sizes names filter

[youknowriad]

closes #8219

Add a new editor config: "availableImageSizes" to be able to display the image sizes labels properly in the image block's inspector.

(Needs to be backported to Core once merged cc @pento)

Testing instructions

• Repeat instructions in Image size control don't display proper labels for sizes. #8219 and verify the size names are displayed properly.

[jorgefilipecosta]

I added the following filter:

function my_custom_sizes( $sizes ) {
    return array_merge( $sizes, array(
        'full' => __('My Custom Full Size'),
    ) );
}

add_filter( 'image_size_names_choose', 'my_custom_sizes' );

And the full label was successfully changed. I think we need to switch id to slug on the editor defaults but besides that things look good to me 👍

[jorgefilipecosta]

I think here we should use slug instead of id, otherwise const availableImageSizesBySlug = keyBy( availableImageSizes, 'slug' );is not going to work.

[jorgefilipecosta]

We already have this array duplicated in many places of the WordPress codebase. When backporting this, it may make sense to check if we can use a constant or another approach to avoid this duplication.

[gziolo]

Out of curiosity, do you have any idea how does it relate to the existing filter for the size of the featured image:

https://github.com/WordPress/gutenberg/blob/master/packages/editor/src/components/post-featured-image/index.js#L33-L43

const mediaSize = applyFilters( 'editor.PostFeaturedImage.imageSize', 'post-thumbnail', media.id, currentPostId );

https://github.com/WordPress/gutenberg/blob/master/docs/extensibility/extending-editor.md#editorpostfeaturedimageimagesize

[youknowriad]

@gziolo It doesn't seem like the filter is directly related. Not that I like the idea of a filter, I think we should remove all frontend filters 🤷‍♂️ but it's addressing another use-case.