Move issue pin to an standalone table for querying performance (#33452)

Noticed a SQL in gitea.com has a bigger load. It seems both `is_pull`
and `pin_order` are not indexed columns in the database.

```SQL
SELECT `id`, `repo_id`, `index`, `poster_id`, `original_author`, `original_author_id`, `name`, `content`, `content_version`, `milestone_id`, `priority`, `is_closed`, `is_pull`, `num_comments`, `ref`, `pin_order`, `deadline_unix`, `created_unix`, `updated_unix`, `closed_unix`, `is_locked`, `time_estimate` FROM `issue` WHERE (repo_id =?) AND (is_pull = 0) AND (pin_order > 0) ORDER BY pin_order
```

I came across a comment
https://github.com/go-gitea/gitea/pull/24406#issuecomment-1527747296
from @delvh , which presents a more reasonable approach. Based on this,
this PR will migrate all issue and pull request pin data from the
`issue` table to the `issue_pin` table. This change benefits larger
Gitea instances by improving scalability and performance.

---------

Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>

models/issues/issue_list.go

@@ -506,6 +506,39 @@ func (issues IssueList) loadTotalTrackedTimes(ctx context.Context) (err error) {
 	return nil
 }
 
+func (issues IssueList) LoadPinOrder(ctx context.Context) error {
+	if len(issues) == 0 {
+		return nil
+	}
+
+	issueIDs := container.FilterSlice(issues, func(issue *Issue) (int64, bool) {
+		return issue.ID, issue.PinOrder == 0
+	})
+	if len(issueIDs) == 0 {
+		return nil
+	}
+	issuePins, err := GetIssuePinsByIssueIDs(ctx, issueIDs)
+	if err != nil {
+		return err
+	}
+
+	for _, issue := range issues {
+		if issue.PinOrder != 0 {
+			continue
+		}
+		for _, pin := range issuePins {
+			if pin.IssueID == issue.ID {
+				issue.PinOrder = pin.PinOrder
+				break
+			}
+		}
+		if issue.PinOrder == 0 {
+			issue.PinOrder = -1
+		}
+	}
+	return nil
+}
+
 // loadAttributes loads all attributes, expect for attachments and comments
 func (issues IssueList) LoadAttributes(ctx context.Context) error {
 	if _, err := issues.LoadRepositories(ctx); err != nil {