The frontend enroll message was display as a `<small/>` tag, but it
seems that lxml HTMLParser parse it wrongly, for example:

  html.tostring(html.fromstring('<small data-oe-model="test"><p></p></small>'))

returns:

  '<div><small data-oe-model="test"></small><p></p></div>'

So branding attributes like data-oe-model that are on small tag are not
found on root node that has become a `div` tag after parsing => this causes
a traceback error when saving a change in this part.

Fix: use small as wrapper for `<div/>` tag that is treated correctly by
HTMLParser (span is also treated correctly but div makes more sense
here).

opw-2573955

closes odoo/odoo#72304

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