set z-index for navbar and sidebar explicitly

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

app/assets/stylesheets/sections/header.scss

@@ -4,6 +4,7 @@
  */
 header {
   &.navbar-gitlab {
+    z-index: 100;
     margin-bottom: 0;
     min-height: 40px;
     border: none;
@@ -82,8 +83,6 @@ header {
     }
   }
 
-  z-index: 10;
-
   .container {
     width: 100% !important;
     padding-left: 0px;

app/assets/stylesheets/sections/sidebar.scss

@@ -3,6 +3,7 @@
 }
 
 .sidebar-wrapper {
+  z-index: 99;
   overflow-y: auto;
   background: #F5F5F5;
 }