[FIX] util/snippets: generate HTML instead of XML for mailing.mailing

[bso-odoo]

The convert_html_content tool writes XML values if a change happened. This is a problem for the fields of mailing.mailing because they strictly expect HTML content. E.g. if <img src="..."> becomes <img src="..."/>, the wysiwyg editor displays an empty template instead of the actual content.

This commit avoids this by making the writes to the fields of mailing.mailing formatted in HTML.

task-3797854

[robodoo]

Pull request status dashboard.

[bso-odoo]

Related to https://github.com/odoo/upgrade/pull/5808