Extracted from #9403

This pull request refactors server-side editor initialization to pass initial edits as a setting directly to the editor initialization, rather than by proxy through a window._wpGutenbergDefaultPost global. In doing so, it resolves a yet-undiscovered issue where our filtering of the_title was effectively disregarded, since we never used the rendered property of the title populating the editor. This will be further separately improved by efforts in #9403.

Related to these future efforts, I think we should consider demo content as an external influencer, exposing filters necessary to provide these initial edits, in a similar fashion to what exists already with the block_editor_settings filter. Here's a patch which, when applied to this branch, would achieve this. However, I am choosing to defer this to after #9403, as it is not my desire for the raw / rendered distinction of content attributes persist, and thus be exposed into a public interface for extending initial edits.

Testing instructions:

Verify there are no regressions in the behavior of initial edits, notably:

• Demo post title and content
• Editing an existing post
• New post with and without the_title filter

Here's a small plugin for testing the_title:

<?php

/**
 * Plugin Name: Filter The Title
 */

add_filter( 'the_title', function() {
	return 'My default title';
} );

(Note: the_title is not an editor-specific filter, so it will have additional side-effects to leave the above plugin activated after testing)