Add support for dangling builds

What does this MR do?

This merge request adds support for dangling builds - jobs that are not associated with pipelines and stages.

What are the relevant issue numbers?

Closes gitlab-org/gitlab-ce#52239

Does this MR meet the acceptance criteria?

• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the database guides