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/migrations/v1_24/v319.go

@@ -0,0 +1,16 @@
+// Copyright 2025 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package v1_24 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddExclusiveOrderColumnToLabelTable(x *xorm.Engine) error {
+	type Label struct {
+		ExclusiveOrder int `xorm:"DEFAULT 0"`
+	}
+
+	return x.Sync(new(Label))
+}