Some refactors for issues stats (#24793)

This PR

- [x] Move some functions from `issues.go` to `issue_stats.go` and
`issue_label.go`
- [x] Remove duplicated issue options `UserIssueStatsOption` to keep
only one `IssuesOptions`

models/issues/issue_update.go

@@ -81,7 +81,7 @@ func doChangeIssueStatus(ctx context.Context, issue *Issue, doer *user_model.Use
 	}
 
 	// Update issue count of labels
-	if err := issue.getLabels(ctx); err != nil {
+	if err := issue.LoadLabels(ctx); err != nil {
 		return nil, err
 	}
 	for idx := range issue.Labels {