Fix empty header

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/assets/stylesheets/generic/header.scss

@@ -8,6 +8,11 @@ header {
   &.navbar-empty {
     background: #FFF;
     border-bottom: 1px solid #EEE;
+
+    .center-logo {
+      margin: 8px 0;
+      text-align: center;
+    }
   }
 
   &.navbar-gitlab {

app/views/layouts/header/_empty.html.haml

 %header.navbar.navbar-fixed-top.navbar-empty
   .container
-    %h4.center
+    .center-logo
       = image_tag 'logo-white.png', width: 32, height: 32