[FIX] website_blog: use viewref when rendering latest posts template
[FIX] website_blog: use viewref when rendering latest posts template

Indeed, a theme can declare its own latest posts template so it would
crash with ref instead of viewref.

Part of https://github.com/odoo/odoo/pull/38510
Related to task-2083198

X-original-commit: 755944f6b88b069515881c9ec58e72e7a0cdd762
ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo