Graphic visualization of `.gitlab-ci.yml`

Description

.gitlab-ci.yml can be tough for a new person on your team to understand. When someone views the file within GitLab, we should provide an option to view a pretty, graphical representation of the file.

Proposal

Links

• Wercker's plugin to Bitbucket: http://blog.wercker.com/2015/12/02/Pipeline-Viewer-For-Bitbucket.html
• #21485 (moved)