[FIX] mass_mailing, web_editor: send mail to outlook and keep image size

Office Outlook doesn't take into account the [style] attribute, and it
doesn't manage width/height in percent value.

This commit set width/heigth attribute in pixels and also always set the
width/height in style attribute (keeping them in the original unit and
setting them to auto if not present).

This is a reintroduction of 2015 commit bdf58adb

opw-2447756

closes odoo/odoo#65212

X-original-commit: c51762a7
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>