& latest xcg/templates/odoo_module.

converter backport in 2021-06.
In particular, add "HTML + mustache" / "MJML + mustache" instead of just "mustache".

Request contents only logged at debug level.

Add constraint on attachment field

Note that it is normally expected that when printing a report from an object,
the created document is saved as an attachment to the object
if the field "Save as attachment prefix" is filled in.

Problem:
This does not work for redner reports (works for QWEB and worked for py3o).

Updated the NEWS.rst file
Version update in dunder file dunder

Redner varlist substitute get_redner_tmpl_keys func for
retrieving redner templates variables.

Rename "Template" -> "Odoo Template"
Rename "Template + Deserialize" -> "Odoo Template + Eval"

Rework on relations converters
Remove deprecated field (ie: deserialize)
Use converter on mail template model
Updated the french translations

Added a custom converter image

This method will be called in ``_render_template`` method to
add additional variables.

Reviewed ir.actions.report and mail template models; Replace
obsolete methode "render_template" by "_render_template"

Retrieve the list of variables present in the template.

Only required for mustache/handlebars template engines.

Following the new functionality (template engine with od+mustache),
this change aims to improve:

* the creation of redner template
* the update of redner template