[FIX] web_kanban: display add button for many2many kanban view

addons/web/static/src/js/views/form_relational_widgets.js

@@ -1967,6 +1967,10 @@ odoo.define('web_kanban.Many2ManyKanbanView', function (require) {
     });
 
     var Many2ManyKanbanView = KanbanView.extend({
+        load_kanban: function(data) {
+            this._super(data);
+            this.render_buttons();
+        },
         get_quick_create_class: function () {
             return Many2ManyQuickCreate;
         },

addons/web_kanban/static/src/xml/web_kanban.xml

@@ -13,7 +13,7 @@
     </div>
 </t>
 <t t-name="KanbanView.buttons">
-    <t t-if="widget.options.action_buttons !== false and widget.is_action_enabled('create')">
+    <t t-if="(widget.options.action_buttons !== false and widget.is_action_enabled('create')) or (!widget.view_id and !widget.options.read_only_mode and (widget.is_action_enabled('write') || widget.is_action_enabled('create')))">
         <div class="oe_kanban_buttons">
             <button type="button" class="oe_kanban_button_new btn btn-primary btn-sm">
                 <t t-esc="widget.options.create_text || _t('Create')"/>