feat(ui): add mark-delivered button on sent postcards with auto today date

app/templates/postcard_detail.html

@@ -5,6 +5,11 @@
     <h1>{{ postcard.card_number }}</h1>
     <div>
         <a href="/postcards/{{ postcard.id }}/edit" class="btn">编辑</a>
+        {% if postcard.status == 'sent' %}
+        <form method="post" action="/postcards/{{ postcard.id }}/mark-delivered" class="inline">
+            <button type="submit" class="btn btn-primary" onclick="return confirm('确认已送达？')">标记已送达</button>
+        </form>
+        {% endif %}
         <form method="post" action="/postcards/{{ postcard.id }}/delete" class="inline">
             <button type="submit" class="btn btn-danger" onclick="return confirm('确定删除？')">删除</button>
         </form>