Add preloading of autosaves for block editor

[talldanwp]

With the introduction of ​https://github.com/WordPress/gutenberg/pull/7945, the block editor requests autosave data when the editor is loaded. This can be optimized by preloading the request server-side and then passing the request data to the client using the preloading mechanism in editor-form-blocks.php.

[aduth]

I expect the Gutenberg change would land as either a bugfix patch release (5.2.x) or minor release (5.3.0), but is not relevant for inclusion in 5.2.0.