Uncouple EpicTreeSorting concern from implementations

What does this MR do?

Currently the implementations of EpicTreeSortingConcern (Epic and EpicIssue) are known by this concern, which they both include.

This circular dependency is problematic since it complicates adding new node types, and violates basic principles of encapsulation (that parents and interfaces do not have knowledge of their implementations).

This MR allows implementations to register themselves, so that we get the same behaviour without up-front knowledge being encoded in the concern.

Screenshots

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.