Use CSS to make items bold in the admin menu

From a semantic point of view, there's no reason to add a strong
emphasis to the menu items.

Besides, using CSS simplifies the code and is less error-prone. For
instance, the "stats" section didn't have a <strong> tag, and so it was
the only one which wasn't bold.

app/assets/stylesheets/admin/menu.scss

@@ -20,6 +20,10 @@
     }
   }
 
+  > ul > li a {
+    font-weight: bold;
+  }
+
   li {
     margin: 0;
     outline: 0;