[FIX] website: missing seo name character

Currently entering custom urls will be missing characters if they contain unicode.

Steps to reproduce:
1. Open Promote dialog
2. Enter a custom url that contains unicode (e.g. `Nội dung có Dấu`)
3. Output: `n-i-dung-c-d-u`

Expected output after this commit: `noi-dung-co-dau`

closes odoo/odoo#104572

Signed-off-by: Romain Derie (rde) <rde@odoo.com>