删除每个页面上单独查询公告的逻辑，改为使用 templatetag 实现。

utils/templatetags/announcement_list.py

@@ -0,0 +1,10 @@
+# coding=utf-8
+from django import template
+
+from announcement.models import Announcement
+
+def public_announcement_list():
+    return Announcement.objects.filter(is_global=True, visible=True).order_by("-create_time")
+
+register = template.Library()
+register.assignment_tag(public_announcement_list, name="public_announcement_list")