(core) Disabling 'Add widget to page' for special pages

Summary: Disabling "Add widget to page" on special pages like acl or code

Test Plan: browser tests

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2969

app/client/models/DocPageModel.ts

@@ -303,7 +303,8 @@ function addMenu(importSources: ImportSource[], gristDoc: GristDoc, isReadonly:
       (elem) => openPageWidgetPicker(elem, gristDoc.docModel, (val) => gristDoc.addWidgetToPage(val).catch(reportError),
                                      {isNewPage: false, selectBy}),
       menuIcon("Widget"), "Add Widget to Page", testId('dp-add-widget-to-page'),
-      dom.cls('disabled', isReadonly)
+      // disable for readonly doc and all special views
+      dom.cls('disabled', (use) => typeof use(gristDoc.activeViewId) !== 'number' || isReadonly),
     ),
     menuItem(() => gristDoc.addEmptyTable().catch(reportError),
       menuIcon("TypeTable"), "Add Empty Table", testId('dp-empty-table'),