[FIX] mass_mailing: make subject translatable
[FIX] mass_mailing: make subject translatable

In f50fbc6d168f0d17fa1e932b4ecc968a92b02b65 the `subject` field was
created and made independent from the `name`, but it must be
translatable, just like the mail body.

ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo,ygol/odoo