Add board-milestone link to Group Export

Currently Group Export has link missing between Board and Milestone.

This is due to predefined MilestoneStruct https://gitlab.com/gitlab-org/gitlab/blob/master/app/models/milestone.rb#L6-10

Group Export fails due to serialisation error when Milestone relation is included in boards. This is because MilestoneStruct does not implement to_json, and there are no db records for these Structs either.

Investigate a way to ignore these default milestones, so that they are not included in the Export, or a way to make export successful (maybe converting MilestoneStruct into a Milestone hash). Or any other solution.