Split migrations folder (#21549)

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

models/migrations/v1_12/v124.go

@@ -0,0 +1,24 @@
+// Copyright 2020 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_12 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddUserRepoMissingColumns(x *xorm.Engine) error {
+	type VisibleType int
+	type User struct {
+		PasswdHashAlgo string      `xorm:"NOT NULL DEFAULT 'pbkdf2'"`
+		Visibility     VisibleType `xorm:"NOT NULL DEFAULT 0"`
+	}
+
+	type Repository struct {
+		IsArchived bool     `xorm:"INDEX"`
+		Topics     []string `xorm:"TEXT JSON"`
+	}
+
+	return x.Sync2(new(User), new(Repository))
+}