Purpose of this commit is to prepare further improvements in template and
composer rendering methods. First step is to
* make them private;
* correctly name parameters (notably rename fields to render_fields to avoid
collision with odoo.fields);
* remove the "single / multi" mode. This is a relic of old implementation
when rendering was done mainly record by record and sometimes by batch.
Now everything should be batched;
* split some lines to ease future diff in upcoming commits;
* improve some docstrings;
Next commits will rewrite and split parts of ``_generate_email``.
Task-2710804 (Mail: Clean MailThread Posting API)
Prepares Task-2088884 (Mail: Use editable computed stored fields in composer)
Part-of: odoo/odoo#99482
Thibault Delavallée
a19f2e3390