[FIX] mass_mailing: never ending resize

In odoo@2972976 a mouseup that unbound resizing in mass mailing was
missing so once resizing there was no way to stop resizing.

addons/mass_mailing/static/src/js/mass_mailing_editor.js

@@ -59,6 +59,10 @@ options.registry["width-x"] = options.Class.extend({
                     self.change_width(event, self.$target, target_width, offset, true);
                 }
             });
+
+            $body.one("mouseup", function(){
+                $body.off('.mass_mailing_width_x');
+            });
         });
     },
     change_width: function (event, target, target_width, offset, grow) {