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/boards/components/issue_card_inner.vue

@@ -149,7 +149,8 @@
         <a
           :href="issue.path"
           :title="issue.title"
-          class="js-no-trigger">{{ issue.title }}</a>
+          class="js-no-trigger"
+          @mousemove.stop>{{ issue.title }}</a>
         <span
           v-if="issueId"
           class="board-card-number append-right-5"