Edit Site: Allow wide alignment (#23488)

For the Site Editor, we want to allow post content to be inserted at full width (e.g. Cover Blocks). This is currently impossible (see Automattic/wp-calypso#43640) due to two independent problems. This PR fixes them.

1. [Unlike the Post Editor](https://github.com/WordPress/WordPress/blob/f18870ae4e1cb3cd81136424e43c99d705aa2912/wp-admin/edit-form-blocks.php#L286), we're currently not setting `alignWide` in the Site Editor's settings, which becomes a problem here: https://github.com/WordPress/gutenberg/blob/9cd07a92d463377767f56c2b3985badbd363d98f/packages/block-editor/src/hooks/align.js#L173-L196.
2. In order to specifically make the Post Content block full-width, we need to add `align` to its `supports` attribute. (It's still conceivable to use this block at other widths, e.g. in a setting with a sidebar.)

Finally, there remained a 10px padding even after applying these changes. This was fixed by removing block list padding from the Site Editor (a change that had previously been applied to the Post Editor in #22213).

lib/edit-site-page.php

@@ -127,6 +127,7 @@ function gutenberg_edit_site_init( $hook ) {
 	}
 
 	$settings = array(
+		'alignWide'              => get_theme_support( 'align-wide' ),
 		'disableCustomColors'    => get_theme_support( 'disable-custom-colors' ),
 		'disableCustomFontSizes' => get_theme_support( 'disable-custom-font-sizes' ),
 		'imageSizes'             => $available_image_sizes,

packages/block-library/src/post-content/block.json

@@ -6,6 +6,7 @@
 		"postType"
 	],
 	"supports": {
+		"align": true,
 		"html": false
 	}
 }

packages/block-library/src/template-part/block.json

@@ -13,6 +13,7 @@
 		}
 	},
 	"supports": {
+		"align": true,
 		"html": false
 	}
 }

packages/edit-site/src/components/block-editor/style.scss

@@ -1,17 +1,3 @@
-.edit-site-block-editor__block-list {
-	padding-bottom: $grid-unit-30;
-	padding-top: $grid-unit-30 + 5;
-
-	padding-left: $block-padding;
-	padding-right: $block-padding;
-
-	// Full-wide. (to account for the padddings added above)
-	.wp-block[data-align="full"] {
-		margin-left: -$block-padding;
-		margin-right: -$block-padding;
-	}
-}
-
 // The button element easily inherits styles that are meant for the editor style.
 // These rules enhance the specificity to reduce that inheritance.
 // This is duplicated in visual-editor.