(core) Populate doc title, description and thumbnail in app.html

Summary:
Fills in the title and description/thumbnail (for templates) in app.html if the
page being requested is for a document.

Test Plan: Tested manually.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3544

package.json

@@ -41,6 +41,7 @@
     "@types/fs-extra": "5.0.4",
     "@types/image-size": "0.0.29",
     "@types/js-yaml": "3.11.2",
+    "@types/jsesc": "3.0.1",
     "@types/jsonwebtoken": "7.2.8",
     "@types/lodash": "4.14.117",
     "@types/lru-cache": "5.1.1",
@@ -113,6 +114,7 @@
     "fs-extra": "7.0.0",
     "grain-rpc": "0.1.7",
     "grainjs": "1.0.2",
+    "handlebars": "4.4.5",
     "highlight.js": "9.13.1",
     "http-proxy-agent": "5.0.0",
     "https-proxy-agent": "5.0.1",
@@ -120,6 +122,7 @@
     "image-size": "0.6.3",
     "jquery": "2.2.1",
     "js-yaml": "3.12.0",
+    "jsesc": "3.0.2",
     "jsonwebtoken": "8.3.0",
     "knockout": "3.5.0",
     "locale-currency": "0.0.2",