actions: Improve format for delete events.

There were several problems with the old format:

* The sender was not necessarily the sender; it was the person who did
  the deletion (which could be an organization administrator)
* It didn't include the ID of the sender, just the email address.
* It didn't include the recipient ID, instead having a semi-malformed
  recipient_type_id under the weird name recipient_user_ids.

Since nothing was relying on the old behavior, we can just fix the
event structure.

zerver/lib/actions.py

@@ -4379,14 +4379,15 @@ def do_delete_messages(user_profile: UserProfile, messages: Iterable[Message]) -
 
         event = {
             'type': 'delete_message',
-            'sender': user_profile.email,
+            'sender': message.sender.email,
+            'sender_id': message.sender_id,
             'message_id': message.id,
             'message_type': message_type, }  # type: Dict[str, Any]
         if message_type == "stream":
             event['stream_id'] = message.recipient.type_id
             event['topic'] = message.topic_name()
         else:
-            event['recipient_user_ids'] = message.recipient.type_id
+            event['recipient_id'] = message.recipient_id
 
         # TODO: Each part of the following should be changed to bulk
         # queries, since right now if you delete 1000 messages, you'll