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/javascripts/behaviors/markdown/highlight_current_user.js

+/**
+ * Highlights the current user in existing elements with a user ID data attribute.
+ *
+ * @param elements DOM elements that represent user mentions
+ */
+export default function highlightCurrentUser(elements) {
+  const currentUserId = gon && gon.current_user_id;
+  if (!currentUserId) {
+    return;
+  }
+
+  elements.forEach(element => {
+    if (parseInt(element.dataset.user, 10) === currentUserId) {
+      element.classList.add('current-user');
+    }
+  });
+}