List epic related issues by relative_position

What does this MR do?

It fixes ordering issues for an epic.

We use find_each method for fetching issues in batched when loading epic related issues. This method uses find_in_batches which supports ordering records only by primary key

We have implemented a custom method for loading batchers which should be the preferred way to load records in batches. Among other things, this method supports the custom ordering of records.

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.

Issue: #36868 (closed)