feat(invite): add invite code registration system

app/templates/base.html

@@ -14,6 +14,9 @@
             {% if user is defined and user %}
             <a href="/profiles">Profile</a>
             <a href="/settings">设置</a>
+            {% if user.is_admin %}
+            <a href="/admin/invites">邀请码管理</a>
+            {% endif %}
             <a href="/logout" class="nav-logout">退出</a>
             {% endif %}
         </div>