Change markdown header tab anchor links to buttons

What does this MR do?

Changes markdown header tab anchor links to buttons. (see https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21988#note_105636972)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

app/assets/stylesheets/framework/secondary_navigation_elements.scss

@@ -8,15 +8,17 @@
   height: auto;
   border-bottom: 1px solid $border-color;
 
-  li {
+  li:not(.md-header-toolbar) {
     display: flex;
 
-    a {
+    a,
+    button {
       padding: $gl-btn-padding;
       padding-bottom: 11px;
       font-size: 14px;
       line-height: 28px;
       color: $gl-text-color-secondary;
+      border: 0;
       border-bottom: 2px solid transparent;
       white-space: nowrap;
 
@@ -33,7 +35,12 @@
       }
     }
 
+    button {
+      padding-top: 0;
+    }
+
     &.active a,
+    &.active button,
     a.active {
       color: $black;
       font-weight: $gl-font-weight-bold;
@@ -42,6 +49,10 @@
         color: $black;
       }
     }
+
+    &.md-header-tab button {
+      line-height: 19px;
+    }
   }
 }