(core) Add initial tutorials implementation

Summary:
Documents can now be flagged as tutorials, which causes them to display
Markdown-formatted slides from a special GristDocTutorial table. Tutorial
documents are forked on open, and remember the last slide a user was on.
They can be restarted too, which prepares a new fork of the tutorial.

Test Plan: Browser tests.

Reviewers: jarek

Reviewed By: jarek

Differential Revision: https://phab.getgrist.com/D3813

app/client/ui/ShareMenu.ts

@@ -44,6 +44,8 @@ export function buildShareMenuButton(pageModel: DocPageModel): DomContents {
         menuOriginal(doc, appModel, true),
         menuExports(doc, pageModel),
       ], {buttonAction: backToCurrent});
+    } else if (doc.isTutorialFork) {
+      return null;
     } else if (doc.isPreFork || doc.isBareFork) {
       // A new unsaved document, or a fiddle, or a public example.
       const saveActionTitle = doc.isBareFork ? t("Save Document") : t("Save Copy");