Only display Featured Image field if theme supports post thumbnails

[aduth]

A Featured Image field should only be available both if the current post type has thumbnail supports (available from post types endpoints) and the theme supports post thumbnails. It is not currently possible to retrieve theme supports from the REST API. Alternatives exist, notably bootstrapping this data server-side on page load, but see #2499 for context on why this is not advisable.

[jeffpaul]

This ticket was mentioned in Slack in #core-editor by jeffpaul. View the logs.

[danielbachhuber]

After #6318 lands, we'll be able to use the theme_supports attribute on the index to expose this data into Gutenberg.

[danielbachhuber]

@aduth @youknowriad @gziolo I'm not quite sure the check itself lives within PostFeaturedImageCheck or PostTypeSupportCheck. Do any of you have an opinion?

And, on that note, it's odd that PostTypeSupportCheck accepts an array of supportKeys. I can't think of a use case for how PostTypeSupportCheck would process multiple values. Currently, it's only ever used in its singular form:

$ ack '<PostTypeSupportCheck'
post-author/check.js
26:	return <PostTypeSupportCheck supportKeys="author">{ children }</PostTypeSupportCheck>;
post-excerpt/check.js
7:	return <PostTypeSupportCheck { ...props } supportKeys="excerpt" />;
post-last-revision/check.js
16:	return <PostTypeSupportCheck supportKeys="revisions" >{ children }</PostTypeSupportCheck>;

[gziolo]

There is one use case where two values are necessary for PostTypeSupportCheck to render its content:

gutenberg/edit-post/components/sidebar/discussion-panel/index.js

Line 17 in c2915d1

<PostTypeSupportCheck supportKeys={ [ 'comments', 'trackbacks' ] }>

PostFeaturedImageCheck only wraps PostTypeSupportCheck which provides the supportKeys value and handles the logic of conditional rendering of all children elements passed down.

[aduth]

Related: #4815, #4812

[aduth]

I can't think of a use case for how PostTypeSupportCheck would process multiple values.

Previously: #4161 (review)

[aduth]

Might we want a new/separate ThemeSupportCheck ? Then use a combination of ThemeSupportCheck and PostTypeSupportCheck in PostFeaturedImageCheck ?