(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

app/client/ui/AppUI.ts

@@ -128,6 +128,10 @@ function pagePanelsDoc(owner: IDisposableOwner, appModel: AppModel, appObj: App)
 
   // Set document title to strings like "DocName - Grist"
   owner.autoDispose(subscribe(pageModel.currentDocTitle, (use, docName) => {
+    // If the document hasn't loaded yet, don't update the title; since the HTML document already has
+    // a title element with the document's name, there's no need for further action.
+    if (!pageModel.currentDoc.get()) { return; }
+
     document.title = `${docName}${getPageTitleSuffix(getGristConfig())}`;
   }));