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/vue_shared/components/markdown/header.vue

@@ -72,6 +72,7 @@
         <button
           class="js-preview-link js-md-preview-button"
           tabindex="-1"
+          @click.prevent="previewMarkdownTab($event)"
         >
           Preview
         </button>