(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/AccountWidget.ts

@@ -81,10 +81,10 @@ export class AccountWidget extends Disposable {
   private _buildUseThisTemplateButton() {
     return cssUseThisTemplateButton(t('Use This Template'),
       dom.attr('href', use => {
-        // Keep the redirect param of the login/signup URL fresh.
-        use(urlState().state);
-        return getLoginOrSignupUrl();
+        const {doc: srcDocId} = use(urlState().state);
+        return getLoginOrSignupUrl({srcDocId});
       }),
+      dom.on('click', () => { this._docPageModel?.clearUnsavedChanges(); }),
       testId('dm-account-use-this-template'),
     );
   }
@@ -109,7 +109,7 @@ export class AccountWidget extends Disposable {
       t("Toggle Mobile Mode"),
       cssCheckmark('Tick', dom.show(viewport.viewportEnabled)),
       testId('usermenu-toggle-mobile'),
-     );
+    );
 
     if (!user) {
       return [