Make RelativePositioning reusable

What does this MR do?

RelativePositioning module was heavily dependent on the Issue model. This changes makes it easier to reuse the functionality provided by RelativePositioning in other models.

This change is needed for an upcoming feature (https://gitlab.com/gitlab-org/gitlab-ee/issues/12196) where we'd like to have drag-and-drop sort functionality.

Does this MR meet the acceptance criteria?

Conformity

• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Performance and testing