Modern Business Full Site Editing - add FSE classes for header and footer (#1211)

* Modern Business  - Full Site Editing - Add  `.fse-enabled` selectors to block css for header and footer if FSE is enabled in order to apply the correct front end block styles to the FSE header and footer content.

modern-business/sass/blocks/_blocks.scss

@@ -1,7 +1,9 @@
 /* !Block styles */
 
 .entry .entry-content > *,
-.entry .entry-summary > * {
+.entry .entry-summary > *, 
+.fse-enabled .site-header > *,
+.fse-enabled .site-footer > * {
 	margin: $size__vertical-spacing-unit 0;
 	max-width: 100%;
 
@@ -96,7 +98,9 @@
 /*
  * Make sure the first block has margin-top: 0
  */
-.entry:not(.has-post-thumbnail) .entry-content > :first-child {
+.entry:not(.has-post-thumbnail) .entry-content > :first-child,
+.fse-enabled .site-header > :first-child,
+.fse-enabled .site-footer > :first-child {
 	margin-top: 0;
 }
 
@@ -106,7 +110,9 @@
  * - helps with plugin compatibility
  */
 .entry .entry-content,
-.entry .entry-summary {
+.entry .entry-summary,
+.fse-enabled .site-header,
+.fse-enabled .site-footer {
 
 	.entry-content,
 	.entry-summary,
@@ -123,7 +129,9 @@
 	}
 }
 
-.entry .entry-content {
+.entry .entry-content,
+.fse-enabled .site-header,
+.fse-enabled .site-footer {
 
 	//! Headers
 	& > h1,