Split migrations folder (#21549)

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

models/migrations/v1_9/v87.go

@@ -0,0 +1,18 @@
+// Copyright 2019 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_9 //nolint
+
+import (
+	"xorm.io/xorm"
+)
+
+func AddAvatarFieldToRepository(x *xorm.Engine) error {
+	type Repository struct {
+		// ID(10-20)-md5(32) - must fit into 64 symbols
+		Avatar string `xorm:"VARCHAR(64)"`
+	}
+
+	return x.Sync2(new(Repository))
+}