[FIX] web_editor: properly handle Bootstrap's rows' negative margins

Bootstrap rows have negative left and right margins, which are not
supported by GMail and Outlook. Sometimes we use padding on columns to
undo the negative margins of their child rows. This corrects these.

This is a backport of commit [1] from 16.0.

[1]: https://github.com/odoo/odoo/commit/b730483f6fd0895241516f032befa8db606b6ba1

Part-of: odoo/odoo#125146