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
