clean code

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

@@ -43,8 +43,6 @@ body.on-edit {
   }
 
   // hide unnecessary elements
-  header,
-  footer,
   .d-edit-none {
     display: none !important;
   }

src/server/views/widget/system-version.html

@@ -1,4 +1,4 @@
-<div class="system-version d-none d-md-block d-print-none">
+<div class="system-version d-none d-md-block d-edit-none d-print-none">
   <span>
     <a href="https://growi.org">GROWI</a> {{ growiVersion() }}
   </span>