Improve issue formatting in Slack service

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

**Before**

![image](https://gitlab.com/jeroenvanbaarsen/gitlab-ce/uploads/670442d4d5cef5aa36753671b8894a75/image.png)

**After**

![image](https://gitlab.com/jeroenvanbaarsen/gitlab-ce/uploads/ab6e40a0c22ebe89ebd5b3652821c6dd/image.png)

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?

See merge request !1308