[IMP] mass_mailing: show blacklist button by default on portal

PURPOSE

Improve and ease subscription and exclusion list portal pages and management

SPECIFICATIONS

We now consider being able to blacklist their email is a default behavior for
users being redirected to the subscription management portal page.

Task-2150462 (Mass Mailing: Unsubscribe flow improvement)

Part-of: odoo/odoo#106568

addons/mass_mailing/data/ir_config_parameter_data.xml

@@ -5,5 +5,9 @@
         <function model="ir.config_parameter"
                   name="set_param"
                   eval="('mass_mailing.mass_mailing_reports', 'True')"/>
+        <!-- Display blacklist button to customers on portal page -->
+        <function model="ir.config_parameter"
+                  name="set_param"
+                  eval="('mass_mailing.show_blacklist_buttons', True)"/>
     </data>
 </odoo>