Purpose is to have all mail template into a mail_template_data.xml file when possible. It eases maintenance and update when having to work globally on template records. Also update some ``body_html`` declarations still using ``xml`` instead of ``html``. Task ID-2534550 (Template usage improvement) Task ID-2477164 (Composer mixin) Prepares Task ID-27033 (QWeb in templates) COM PR odoo/odoo#70889 ENT PR odoo/enterprise#18352