[IMP] mass_mailing: added unsubscribe link in plaintext theme
[IMP] mass_mailing: added unsubscribe link in plaintext theme

As we are hiding snippet bar in plaintext theme, there is no way to add snippet
with unsubscribe link. So we are adding it by default in plaintext theme

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