feat: Add sorting by exclusive labels (issue priority) (#33206)

Fix #2616

This PR adds a new sort option for exclusive labels.

For exclusive labels, a new property is exposed called "order", while in
the UI options are populated automatically in the `Sort` column (see
screenshot below) for each exclusive label scope.

---------

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

models/issues/label.go

@@ -87,6 +87,7 @@ type Label struct {
 	OrgID           int64 `xorm:"INDEX"`
 	Name            string
 	Exclusive       bool
+	ExclusiveOrder  int `xorm:"DEFAULT 0"` // 0 means no exclusive order
 	Description     string
 	Color           string `xorm:"VARCHAR(7)"`
 	NumIssues       int
@@ -236,7 +237,7 @@ func UpdateLabel(ctx context.Context, l *Label) error {
 	}
 	l.Color = color
 
-	return updateLabelCols(ctx, l, "name", "description", "color", "exclusive", "archived_unix")
+	return updateLabelCols(ctx, l, "name", "description", "color", "exclusive", "exclusive_order", "archived_unix")
 }
 
 // DeleteLabel delete a label