Filter out inaccessible issue epics

What does this MR do and why?

When exporting a project, it's possible that some epics referenced by issues won't be accessible by user (who is exporting the project). To make this possible, a new include_if_exportable YML declaration is added so we can use declarative approach for defining conditional export of associations (IOW export some association only if some condition is met).

Related to #371074 (closed)

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