Fix issues missing on epic's page after project import

What does this MR do?

This MR fixes issues being missing on Epic page after project import. The issues were associated with an epic correctly, but were not shown on Epic page, because epic_issue relation had relative_position: nil. Now epic_issue is exported, that contains epic, which will preserve relative_position, which is required in order for the issue to be shown on Epic's page.

Screenshots

Before

After

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.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

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