centering wiki preview

src/client/styles/scss/_on-edit.scss

@@ -238,6 +238,7 @@ body.on-edit {
     .page-editor-preview-body {
       max-width: 980px;
       padding: 18px 15px 0;
+      margin: 0 auto;
     }
 
     .grw-editor-configuration-dropdown {