Change markdown header tab anchor links to buttons

What does this MR do?

EE port of https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21988.

app/assets/stylesheets/framework/secondary_navigation_elements.scss

@@ -8,12 +8,13 @@
   height: auto;
   border-bottom: 1px solid $border-color;
 
-  li {
+  li:not(.md-header-toolbar) {
     display: flex;
 
     a,
     button {
       padding: $gl-btn-padding;
+      padding-bottom: 11px;
       font-size: 14px;
       line-height: 28px;
       color: $gl-text-color-secondary;
@@ -49,6 +50,10 @@
       }
     }
 
+    &.md-header-tab button {
+      line-height: 19px;
+    }
+
     &.md-header-tab.active button {
       border-bottom: 2px solid $theme-gray-800;
     }