Handle Project+Milestone link in Project Export/Import

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In #196152 (closed) we added the link between Board and Milestone to the group export configuration so that this relationship can be exported and imported.

We should now also be able to do this for the link between Project and Milestone, so that this can also be exported/imported for the project export.

Note: we currently export user generated milestones and predefined milestones but we only support the import of user generated ones for now - predefined ones will be ignored.