Issue-3480116-always-keep-homepage: Added functionality to .module file for...

Issue-3480116-always-keep-homepage: Added functionality to .module file for checking if the page being deleted is homepage/403/404 and denying access to delete them.

Closes #3480116

config/schema/node_keep.schema.yml

@@ -3,6 +3,9 @@ node_keep.settings:
   type: config_object
   label: 'Node Keep settings'
   mapping:
+    protect_404_403_front:
+      type: boolean
+      label: 'Enable Node Keep functionality'
     hide_warning_messages:
       type: boolean
       label: 'Hide warning messages'