(core) Add support for auto-copying docs on signup

Summary:
The new "copyDoc" query parameter on the login page sets a short-lived cookie, which is
then read when welcoming a new user to copy that document to their Home workspace, and
redirect to it. Currently, only templates and bare forks set this parameter.

A new API endpoint for copying a document to a workspace was also added.

Test Plan: Browser tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3992

app/client/ui/App.ts

@@ -172,10 +172,9 @@ export class App extends DisposableWithEvents {
     G.window.addEventListener('beforeunload', (ev: BeforeUnloadEvent) => {
       if (unsavedChanges.haveUnsavedChanges()) {
         // Following https://developer.mozilla.org/en-US/docs/Web/API/Window/beforeunload_event
-        const msg = 'You have some unsaved changes';
-        ev.returnValue = msg;
+        ev.returnValue = true;
         ev.preventDefault();
-        return msg;
+        return true;
       }
       this.dispose();
     });