roost: Edited messages should only use message_id for content, not message_ids

[dehnert]

As described in a PR, soon the docs, and discussed on CZO, the message_ids field is intended for the stream+topic, and only message_id should be used for the content.

This probably wants to be fixed in snipe/zulip.py, in the update function (line 358 or so), or perhaps the caller and split update into separate functions for stream/topic and content.