Fix N+1 queries loading milestones when exporting CSVs

During exporting of CSVs, we weren't preloading milestones, so every issue had to load its related milestones.

We already do this for merge requests: https://gitlab.com/gitlab-org/gitlab/blob/2f968c79e8f781951856f71c4e7cd46b35304bf4/app%2Fmodels%2Fmerge_request.rb#L282