Improve Slack notification on Pipeline status

Now we're dogfooding this integration I've seen that much can be improved on this message, and I've created a proposal JSON in $1662408. Preview

I'm not at all UX so please provide feedback, my ideas:

1. Sidebar color reflects the status, red => failed, orange => succeded with warnings, green => succeeded.
2. Author is the user that triggered the pipeline
3. We might want to show how many jobs failed?
4. Show the stage in which the jobs failed
5. Title links to pipeline#show view
6. Show the commit title
7. Timestamp should be set to the pipeline.finished_at property

/cc @pedroms @grzesiek