Add url_code to res_lang that overrides the locale code when used in URL...

In the Odoo CMS, the locale code is used in the website URL to specify a non-default language. However, when migrating from e-commerce platforms (Magento in my case) that use a store-view/subfolder type URL convention, this change would allow more control over URL customizations and preserve original URLs rather than forcing rewrites and/or redirects.

This patch allows the webmaster to override the locale code with a custom "url_code" if present, which adds a subfolder-type alias as a "url_code" in Odoo website URLs.