(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

package.json

@@ -46,6 +46,7 @@
     "@types/chai-as-promised": "7.1.0",
     "@types/content-disposition": "0.5.2",
     "@types/diff-match-patch": "1.0.32",
+    "@types/dompurify": "2.4.0",
     "@types/double-ended-queue": "2.1.0",
     "@types/express": "4.16.0",
     "@types/form-data": "2.2.1",
@@ -59,6 +60,7 @@
     "@types/jsonwebtoken": "7.2.8",
     "@types/lodash": "4.14.117",
     "@types/lru-cache": "5.1.1",
+    "@types/marked": "4.0.8",
     "@types/mime-types": "2.1.0",
     "@types/minio": "7.0.15",
     "@types/mocha": "5.2.5",
@@ -130,6 +132,7 @@
     "cookie-parser": "1.4.3",
     "csv": "4.0.0",
     "diff-match-patch": "1.0.5",
+    "dompurify": "3.0.0",
     "double-ended-queue": "2.1.0-0",
     "exceljs": "4.2.1",
     "express": "4.16.4",
@@ -152,6 +155,7 @@
     "knockout": "3.5.0",
     "locale-currency": "0.0.2",
     "lodash": "4.17.21",
+    "marked": "4.2.12",
     "minio": "7.0.32",
     "moment": "2.29.4",
     "moment-timezone": "0.5.35",