[FIX] base, web_editor, mail, digest: preserve comments in sent e-mails

For email design in the context of Microsoft Outlook, we want to keep
some magic Microsoft comments (Outlook conditional comment), which -
until this commit - were skipped by QWeb. These allow us to change the
rendering exclusively for Outlook so as to overcome some of its
limitations. This commit introduces a qweb rendering option
(`preserve_comments`) for when - like in mass mailing and digest - we
want to keep comments.

Part-of: odoo/odoo#80621