Add context to at mention missed messages

When you are at mentioned in a stream we will now send you up to the
last five messages which were sent in the past 5 minutes on the same
topic and stream.

(imported from commit 6df6c1cf868722a7bf76e54710e38741a7ac8f31)

zerver/models.py

@@ -23,6 +23,7 @@ import zlib
 
 from bitfield import BitField
 from collections import defaultdict
+from datetime import timedelta
 import pylibmc
 import re
 import ujson
@@ -968,6 +969,15 @@ def pre_save_message(sender, **kwargs):
         message = kwargs['instance']
         message.update_calculated_fields()
 
+def get_context_for_message(message):
+    return Message.objects.filter(
+        recipient_id=message.recipient_id,
+        subject=message.subject,
+        id__lt=message.id,
+        pub_date__gt=message.pub_date - timedelta(minutes=5),
+    ).order_by('-id')[:5]
+
+
 class UserMessage(models.Model):
     user_profile = models.ForeignKey(UserProfile)
     message = models.ForeignKey(Message)