Track a user who creates a Pipeline

What does this MR do?

This adds additional column to pipelines to track user who is creating pipelines.

Why was this MR needed?

This is to make it possible to show all pipelines created by specific user and to later solve: https://gitlab.com/gitlab-org/gitlab-ce/issues/18054

What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/18992

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together