Fix overdue marking of closed issues and milestones (#14923)

Closed milestones and issues should only be marked overdue if they were
closed after their deadline.

Fix: #14536

Signed-off-by: Andrew Thornton <art27@cantab.net>

models/issue_milestone.go

@@ -63,8 +63,10 @@ func (m *Milestone) AfterLoad() {
 	}
 
 	m.DeadlineString = m.DeadlineUnix.Format("2006-01-02")
-	if timeutil.TimeStampNow() >= m.DeadlineUnix {
-		m.IsOverdue = true
+	if m.IsClosed {
+		m.IsOverdue = m.ClosedDateUnix >= m.DeadlineUnix
+	} else {
+		m.IsOverdue = timeutil.TimeStampNow() >= m.DeadlineUnix
 	}
 }