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.go

@@ -97,6 +97,9 @@ func (issue *Issue) loadTotalTimes(e Engine) (err error) {
 
 // IsOverdue checks if the issue is overdue
 func (issue *Issue) IsOverdue() bool {
+	if issue.IsClosed {
+		return issue.ClosedUnix >= issue.DeadlineUnix
+	}
 	return timeutil.TimeStampNow() >= issue.DeadlineUnix
 }