Split migrations folder (#21549)

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

models/migrations/v1_7/v74.go

@@ -0,0 +1,16 @@
+// Copyright 2018 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_7 // nolint
+
+import "xorm.io/xorm"
+
+func AddApprovalWhitelistsToProtectedBranches(x *xorm.Engine) error {
+	type ProtectedBranch struct {
+		ApprovalsWhitelistUserIDs []int64 `xorm:"JSON TEXT"`
+		ApprovalsWhitelistTeamIDs []int64 `xorm:"JSON TEXT"`
+		RequiredApprovals         int64   `xorm:"NOT NULL DEFAULT 0"`
+	}
+	return x.Sync2(new(ProtectedBranch))
+}