Fix navbar spacing when "sticky-header" is enabled

InvenTree/InvenTree/static/css/inventree.css

@@ -141,6 +141,10 @@
     float: left;
 }
 
+.navbar-spacer {
+    height: 40px;
+}
+
 #navbar-barcode-li {
     border-left: none;
     border-right: none;

InvenTree/templates/navbar.html

@@ -111,3 +111,7 @@
 
   </div>
 </nav>
+
+{% if sticky %}
+<div class='navbar-spacer'></div>
+{% endif %}