Add support for relative ordering between classes for epics and epic_issues

What does this MR do?

It adds support for ordering epics and epic issues even among themselves when using graphQL mutation. The ordering separately still works.

It also moves all epic_issues after epics (which is the current state in epic_tree) in a migration.

Migration

We have about

• 12000 epics
• 33000 epic_issues

We schedule epics in batches of 100 with 5 minutes delay, which is 120x5 = 600 minutes = 10 hours.

We schedule batches of epics in order to avoid user reordering issues of an epic and updating issues of the same epic in different batches.

Does this MR meet the acceptance criteria?

Conformity

• [-] Changelog entry
• [-] Documentation created/updated or follow-up review issue created
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• [-] Separation of EE specific content

Performance and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #13633 (closed)