[FIX] crm: default subscription

When a CRM activity is defined, the subscription is activated by default
for any user. There is no way to change it since the field is not in
the form view. Therefore, a customer is automatically subscribed to any
internal activity, and the user must manually remove the subscription
afterwards.

The `default` field is displayed in the forw view, so this default
behavior can now be configured for a given activity.

opw-672453

addons/crm/views/crm_action_views.xml

@@ -25,6 +25,7 @@
                         <field name="team_id"/>
                         <field name="description"/>
                         <field name="days"/>
+                        <field name="default"/>
                     </group>
                     <group>
                         <field name="activity_1_id"/>