Via API, users can now access messages which are in web-public
streams without any authentication.
If the user is not authenticated, we assume it is a web-public
query and add `streams:web-public` narrow if not already present
to the narrow. web-public streams are also directly accessible.
Any malformed narrow which is not allowed in a web-public query
results in a 400 or 401. See test_message_fetch for the allowed
queries.