[FIX] addons: no unsubscribe for unsubscribed

The `/groups` route display mail.channel and allow:

- a public user to subscribe specifying a mail address
- a logged in user to subscribe/unsubscribe itself

The `/groups?unsubscribe` route is the same but only allow:

- a public user to unsubscribe specifying a mail address
- a logged in user to unsubscribe itself

When a user is logged in, there is the oddity that all mail.channel
appear as to "unsubscribe" even if the user is not subscribed.

With this commit, the unsubscribe is shown only if user is subscribed.

opw-1861208
closes #25439

addons/website_mail_channel/static/src/js/website_mail_channel.snippet.js

@@ -8,6 +8,7 @@ sAnimation.registry.follow_alias = sAnimation.Class.extend({
     start: function () {
         var self = this;
         this.is_user = false;
+        var unsubscribePage = window.location.search.slice(1).split('&').indexOf("unsubscribe") >= 0;
         this._rpc({
             route: '/groups/is_member',
             params: {
@@ -19,6 +20,9 @@ sAnimation.registry.follow_alias = sAnimation.Class.extend({
             self.is_user = data.is_user;
             self.email = data.email;
             self.$target.find('.js_mg_link').attr('href', '/groups/' + self.$target.data('id'));
+            if (unsubscribePage && self.is_user) {
+                self.$target.find(".js_mg_follow_form").remove();
+            }
             self.toggle_subscription(data.is_member ? 'on' : 'off', data.email);
             self.$target.removeClass("hidden");
         });