IMP: Add single menu for content browsing

* Categories and content are now available and browsable in a single menu.
* Navigation helps user to find content easily
* Cleanups for pylint and flake

document_page/static/src/js/document_page_kanban.js

@@ -0,0 +1,28 @@
+odoo.define("document_page.update_kanban", function(require) {
+    "use strict";
+    
+    var KanbanRecord = require("web.KanbanRecord");
+
+    KanbanRecord.include({
+        // --------------------------------------------------------------------------
+        // Private
+        // --------------------------------------------------------------------------
+
+        /**
+         * @override
+         * @private
+         */
+        _openRecord: function() {
+            if (
+                this.modelName === "document.page" &&
+                this.$(".o_document_page_kanban_boxes a").length
+            ) {
+                this.$(".o_document_page_kanban_boxes a")
+                    .first()
+                    .click();
+            } else {
+                this._super.apply(this, arguments);
+            }
+        }
+    });
+});