Improve issue formatting in Slack service

This will improve the looks of the message that gets send to Slack.

Before

After

There is one thing I'm not certain about, thats sending the logo with the payload. Because this means we need to host the GitLab logo somewhere public, and every time a payload from any GitLab instance is send, we use that logo url. This might overload the server, or drive the bandwidth bill through the roof. This is something @sytses or @dzaporozhets have to decide about.

@stanhu, you told me you were the author of the original code, maybe you can review?