streams: Refactor multi-option helpers into separate functions.

For internal stream messages, most of the time, we have access to
a Stream object. For the few corner cases where we don't, it is a
much cleaner approach to have a separate function that accepts a
stream name than having one multi-option helper that accepts both
names and objects.

zilencer/management/commands/add_mock_conversation.py

@@ -78,9 +78,8 @@ From image editing program:
         ]  # type: List[Dict[str, Any]]
 
         messages = [internal_prep_stream_message(
-            realm, message['sender'],
-            'message formatting', message['content'],
-            stream=stream
+            realm, message['sender'], stream,
+            'message formatting', message['content']
         ) for message in staged_messages]
 
         message_ids = do_send_messages(messages)