(core) Add behavioral and coaching call popups

Summary:
Adds a new category of popups that are shown dynamically when
certain parts of the UI are first rendered, and a free coaching
call popup that's shown to users on their site home page.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3706

app/client/ui/RightPanel.ts

@@ -537,7 +537,7 @@ export class RightPanel extends Disposable {
     const gristDoc = this._gristDoc;
     const section = gristDoc.viewModel.activeSection;
     const onSave = (val: IPageWidget) => gristDoc.saveViewSection(section.peek(), val);
-    return (elem) => { attachPageWidgetPicker(elem, gristDoc.docModel, onSave, {
+    return (elem) => { attachPageWidgetPicker(elem, gristDoc, onSave, {
       buttonLabel:  t('Save'),
       value: () => toPageWidget(section.peek()),
       selectBy: (val) => gristDoc.selectBy(val),