Split migrations folder (#21549)

There are too many files in `models/migrations` folder so that I split
them into sub folders.

models/migrations/v1_17/v211.go

@@ -0,0 +1,26 @@
+// Copyright 2022 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package v1_17 // nolint
+
+import (
+	"fmt"
+
+	"xorm.io/xorm"
+)
+
+func CreateForeignReferenceTable(x *xorm.Engine) error {
+	type ForeignReference struct {
+		// RepoID is the first column in all indices. now we only need 2 indices: (repo, local) and (repo, foreign, type)
+		RepoID       int64  `xorm:"UNIQUE(repo_foreign_type) INDEX(repo_local)" `
+		LocalIndex   int64  `xorm:"INDEX(repo_local)"` // the resource key inside Gitea, it can be IssueIndex, or some model ID.
+		ForeignIndex string `xorm:"INDEX UNIQUE(repo_foreign_type)"`
+		Type         string `xorm:"VARCHAR(16) INDEX UNIQUE(repo_foreign_type)"`
+	}
+
+	if err := x.Sync2(new(ForeignReference)); err != nil {
+		return fmt.Errorf("Sync2: %w", err)
+	}
+	return nil
+}