Fix issue sidebar top position after header height change

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

app/assets/stylesheets/framework/header.scss

@@ -6,12 +6,12 @@ header {
   transition-duration: .3s;
   &.navbar-empty {
-    height: 58px;
+    height: $header-height;
     background: #fff;
     border-bottom: 1px solid $btn-gray-hover;
     .center-logo {
-      margin: 11px 0;
+      margin: 8px 0;
       text-align: center;
       #tanuki-logo, img {

app/assets/stylesheets/pages/issuable.scss

@@ -125,7 +125,7 @@
 .right-sidebar {
   position: fixed;
-  top: 58px;
+  top: $header-height;
   bottom: 0;
   right: 0;
   z-index: 10;