[FIX] mail: use base_url helper to be multi website compliant

When an email is sent and is related to a record, there is a header with a
button link to access the record.

Before this commit, that button href was always linking to the base_url URL.

That was an issue on multi-website, since we need to use the website url and
not the base_url if the record is related to a website.
That's the case for a SO eg, after checkout you receive a confirmation email.
In a multi-website environment, we need to access the record website.

To do so, a previous imp/fix was done with c1be1993.
But this occurence still needed to be replaced.

Fixes #34309

closes odoo/odoo#34397

Signed-off-by: Jérémy Kersten (jke) <jke@openerp.com>