[FIX] base: mail signature CSS inlined for icons etc

The mail signature of an user did not inline the CSS like the body of a
message, so for example icons seemed to work but would not be seen
(unless the mail reader had the same font-awesome system).

With this change we inline the signature too.

On edition there was also an issue since the original icon was saved as
data-class which was stripped: so eg. when editing a mail.message with
working icon, the icons would disappear.

opw-2453912

closes odoo/odoo#66216

X-original-commit: 29347ceefa1f83a350a544a02d040e69f2906aaf
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>