(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/sanitizeHTML.ts

@@ -0,0 +1,26 @@
+import DOMPurify from 'dompurify';
+
+const config = {
+  ADD_TAGS: ['iframe'],
+  ADD_ATTR: ['allowFullscreen'],
+};
+
+DOMPurify.addHook('uponSanitizeAttribute', (node) => {
+  if (!('target' in node)) { return; }
+
+  node.setAttribute('target', '_blank');
+});
+DOMPurify.addHook('uponSanitizeElement', (node, data) => {
+  if (data.tagName !== 'iframe') { return; }
+
+  const src = node.getAttribute('src');
+  if (src?.startsWith('https://www.youtube.com/embed/')) {
+    return;
+  }
+
+  return node.parentNode?.removeChild(node);
+});
+
+export function sanitizeHTML(source: string | Node): string {
+  return DOMPurify.sanitize(source, config);
+}