[IMP] website_slide: subject of publish emails
[IMP] website_slide: subject of publish emails

When publishing a slide, a message is posted using a template defined on the
channel. This commit uses the template subject to form the message
subject instead of having a generic 'Re: <slide>' as subject.

ygol/odoo,hip-odoo/odoo,dfang/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,hip-odoo/odoo,ygol/odoo,ygol/odoo,hip-odoo/odoo,dfang/odoo,dfang/odoo,dfang/odoo