REST API: Introduce Autosaves controller and endpoint.

- Adds `WP_REST_Autosaves_Controller` which extends `WP_REST_Revisions_Controller`.
- Autosaves endpoint is registered for all post types except attachment because even post types without revisions enabled are expected to autosave.
- Because setting the `DOING_AUTOSAVE` constant pollutes the test suite, autosaves tests are run last. We may want to improve upon this later. 

Also, use a truly impossibly high number in User Controller tests. The number `100`, (or `7777` in `trunk`), could be valid in certain test run configurations. The `REST_TESTS_IMPOSSIBLY_HIGH_NUMBER` constant is impossibly high for this very reason.

Finally, Skip Autosaves controller test for multisite. There's a PHP 5.2 edge case where paths calculated differently, possibly caused by differing version of PHPUnit.

Props adamsilverstein, aduth, azaozz, danielbachhuber, rmccue, danielbachhuber.

Merges [43767], [43768], [43769] to trunk.

See #45132, #45131.
Fixes #45128, #43316.
Built from https://develop.svn.wordpress.org/trunk@44126


git-svn-id: http://core.svn.wordpress.org/trunk@43956 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/rest-api.php

@@ -193,6 +193,11 @@ function create_initial_rest_routes() {
 			$revisions_controller = new WP_REST_Revisions_Controller( $post_type->name );
 			$revisions_controller->register_routes();
 		}
+
+		if ( 'attachment' !== $post_type->name ) {
+			$autosaves_controller = new WP_REST_Autosaves_Controller( $post_type->name );
+			$autosaves_controller->register_routes();
+		}
 	}
 
 	// Post types.