[IMP] mass_mailing: warn when unsubscribing

Current behavior before PR:
When a lead or res.partner click in the 'unsubscribe' link in a mail that was sent to him via the mass mailing module (not via a particular mailing list), he is blacklisted from the whole mass mailing system. The only message he gets is "you have been successfully unsubscribed".

Desired behavior after PR is merged:
The user is told that he was unsubscribed, but in addition a warning is displayed telling him explicitly :
 - to which mailing lists he was subscribed
 - that he won't receive any mail from them anymore.

Purpose : Warn the user that unsubscribe via a lead or res.partner mass mailing that he won't receive any mail from the mailing lists (listed in the warning) he had subscribed, if any.

closes odoo/odoo#27678

addons/mass_mailing/controllers/main.py

@@ -62,6 +62,10 @@ class MassMailController(http.Controller):
                     'show_blacklist_button': request.env['ir.config_parameter'].sudo().get_param('mass_mailing.show_blacklist_buttons'),
                 })
             else:
+                opt_in_lists = request.env['mail.mass_mailing.list_contact_rel'].sudo().search([
+                    ('contact_id.email', '=', email),
+                    ('opt_out', '=', False)
+                ]).mapped('list_id')
                 blacklist_rec = request.env['mail.blacklist'].sudo()._add(email)
                 self._log_blacklist_action(
                     blacklist_rec, mailing_id,
@@ -70,6 +74,7 @@ class MassMailController(http.Controller):
                     'email': email,
                     'mailing_id': mailing_id,
                     'res_id': res_id,
+                    'list_ids': opt_in_lists,
                     'show_blacklist_button': request.env['ir.config_parameter'].sudo().get_param(
                         'mass_mailing.show_blacklist_buttons'),
                 })

addons/mass_mailing/views/unsubscribe_templates.xml

@@ -71,6 +71,19 @@
                         <p>You have been successfully <strong>unsubscribed</strong>!</p>
                     </div>
 
+		    <div t-if="list_ids" class="alert alert-warning">
+		        <p class="text-center">You are currently subscribed to those newsletters. Unsubscribing means you will not receive any news from them anymore:</p>
+		        <ul class="list-group mb-4">
+			    <t t-foreach="list_ids" t-as="list_id">
+				<t t-if="list_id.is_public == True">
+				    <li class="list-group-item bg-transparent">
+					<strong><t t-esc="list_id.name"/></strong>
+				    </li>
+				</t>
+			    </t>
+			</ul>
+		    </div>
+
                     <div t-if="show_blacklist_button" class="mb64">
                         <div class="btn btn-secondary pull-right" id="button_add_blacklist" style="display:none">Blacklist Me</div>
                         <div class="btn btn-secondary pull-right" id="button_remove_blacklist" style="display:none">Come Back</div>