Add Message.has_recipient() method.

rattail/db/model/messages.py

@@ -74,6 +74,16 @@ class Message(Base):
         kwargs['recipient'] = user
         self.recipients.append(MessageRecipient(**kwargs))
 
+    def has_recipient(self, user):
+        """
+        Returns boolean indicating whether the given user is listed among the
+        message's recipients.
+        """
+        for recipient in self.recipients:
+            if recipient.recipient is user:
+                return True
+        return False
+
 
 class MessageRecipient(Base):
     """