Populate Message extras url

[jayvdb]

The slack backend provides a Message.extras['url'] , which is a link to the message

        msg.extras['url'] = 'https://{domain}.slack.com/archives/{channelid}/p{ts}'.format(
            domain=self.sc.server.domain,
            channelid=channel_link_name,
            ts=self._ts_for_message(msg).replace('.', '')
        )

I've created a patch for Gitter backend to do the same : errbotio/err-backend-gitter#32

And ideally Zulip does as well.
It is reasonably easy to create the URL for a topic of a stream, but I suspect that the /near/id part is not available in the API. At least I couldnt find it quickly.

[timabbott]

@jayvdb you need the message ID (which should be available in the message object from the Zulip API) and then you can construct a URL with a /#narrow/stream/foo/topic/bar/near/ID structure (the operator/operand pairs can be in any order)