[FIX] website_mail_channel: number of members in channel

The members of a channel are stored in channel_partner_ids, they are no longer
a follower of the object (message_follower_ids)

addons/website_mail_channel/views/website_mail_channel.xml

@@ -50,7 +50,7 @@
                     <div t-esc="group.description" class="text-muted"/>
                 </div>
                 <div class="col-md-2">
-                    <i class='fa fa-fw fa-user'/> <t t-esc="len(group.message_follower_ids)"/> members<br />
+                    <i class='fa fa-fw fa-user'/> <t t-esc="len(group.channel_partner_ids)"/> members<br />
                     <i class='fa fa-fw fa-envelope-o'/> <t t-raw="group_data[group.id]['monthly_message_nbr']"/> messages / month
                 </div>
                 <div class="col-md-3">