Build legacy CSS via cssbundling as well

Add vanilla Sass builds that are also handled by cssbundling-rails.

app/assets/stylesheets/legacy/_layout.scss

@@ -55,6 +55,30 @@
   }
 }
 
+body#admin-panel {
+  #wrapper {
+    > header {
+      background: $color-red-bright;
+      background: linear-gradient(35deg, rgba(255,0,255,0.2) 0, rgba(153,12,14,0.9) 100%),
+                  url('/img/bg-1.jpg');
+    }
+  }
+
+  #main-nav {
+    ul {
+      grid-template-columns: repeat(4, 1fr);
+
+      li {
+        a {
+          &.active {
+            border-bottom: 2px solid $color-red-bright;
+          }
+        }
+      }
+    }
+  }
+}
+
 .flash-msg {
   width: 100%;
   text-align: center;