Always register the autosaves controller

[danielbachhuber]

Description

Ensures the autosaves controller is always registered for post types with show_in_rest=>true. While revisions can be enabled/disabled on a post type by post type basis, the editor expects that a post type can always be autosaved.

Fixes #7442
Fixes #7348

How has this been tested?

1. Disable revisions for the Post post type:

add_action( 'init', function(){
	remove_post_type_support( 'post', 'revisions' );
});

2. In the Gutenberg editor, create a new Post, enter a title, and hit "Preview". You should see the preview of the post display.

Types of changes

Bug fix (non-breaking change which fixes an issue).

Checklist:

• My code is tested.
• My code follows the WordPress code style.
• My code follows the accessibility standards.
• My code has proper inline documentation.

[danielbachhuber]

@adamsilverstein beat me to it in #8682