Merge branch 'mr-button-color' into 'master'

Change merge request button color based on CI status

Green button looks confusing when CI fails

Screenshots:

![ci-warn](https://gitlab.com/gitlab-org/gitlab-ce/uploads/f8166c9acf35f9d886f37f52f975acfb/ci-warn.png)
![ci-can](https://gitlab.com/gitlab-org/gitlab-ce/uploads/d7319c4c567c42a47d79953191384f96/ci-can.png)

See merge request !448