Eager load to avoid N+1 queries when there are labels from multiple projects in a milestone

Open Stan Hu requested to merge sh-eager-load-project-labels-in-milestone into master

Found by Bullet in !10263 (merged) in https://gitlab.com/gitlab-org/gitlab-ce/builds/13490762

This one is subtle: even though we have a QueryRecorder checking for N+1 queries, what we don't test is labels in multiple projects of a milestone.