Similar in the 13.0 branch: 7cc12fb73390

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

add /api/v1 in the code.

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).

Added the .hgconf file

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

Redner varlist substitute get_redner_tmpl_keys func for
retrieving redner templates variables.