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
closesodoo/odoo#66372
X-original-commit: 5e093b7a37984c8cc5aa73d1c0f7aa9c9345e6b1
Signed-off-by: Nicolas Lempereur (nle) <nle@odoo.com>