add model.NewX funcs for builtin types (#7692)

* add model.NewX funcs for builtin types

* whoops, forgot to add the new file

model/builtin.go

@@ -0,0 +1,9 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package model
+
+func NewBool(b bool) *bool       { return &b }
+func NewInt(n int) *int          { return &n }
+func NewInt64(n int64) *int64    { return &n }
+func NewString(s string) *string { return &s }