Load milestone issues tab async
Milestones issues are being rendered inline for milestones show page, with some queries being made on haml, check https://gitlab.com/gitlab-org/gitlab/blob/master/app/views/shared/milestones/_issues_tab.html.haml#L6.
Merge requests, participants and label tabs data are already loaded async. we should do the same for issues.
This is a follow up of #39453 (closed)