Add admin layout with admin navigation

And remove the hacky link list from the dashboard.

app/assets/stylesheets/admin.scss

@@ -0,0 +1,22 @@
+@import "variables";
+
+body#admin-panel {
+  #wrapper {
+    > header {
+      background: $color-red-bright;
+      background: linear-gradient(35deg, $color-purple 0, $color-red-dark 100%);
+    }
+  }
+
+  #main-nav {
+    ul {
+      li {
+        a {
+          &.active {
+            border-bottom: 2px solid $color-red-bright;
+          }
+        }
+      }
+    }
+  }
+}