Messages are saved with null acl that allow public read

[Steven-Chan]

Related issues:

• Migrate to use role base ACL for message #108
• Role based ACL migration #119

Related code:

• https://github.com/SkygearIO/chat/blob/master/chat/message_handlers.py#L152
• https://github.com/SkygearIO/chat/blob/master/chat/conversation.py#L126

It was expected that only participants of the conversation have read and write permission to the messages.

However, it has been found that newly created messages have _access to be NULL, which would allow public to read.