Name downloaded books with a template

The new "Book Downloads" group in Settings holds a Calibre template for the file name of a
direct download. Without a template, the name stays the title. The name is also the title if
the template fails or gives no name for a book.

tests/js/auth-flows.test.mjs

@@ -11,7 +11,7 @@ test('login retry, navigation, session expiry and logout preserve the authentica
   const backend = server(t);
   let auth = { authenticated: false, mode: 'cwa', user: null };
   backend.on('GET', 'auth/check', () => auth);
-  backend.on('GET', 'config', { metadata_fields: {}, mode_labels: {}, library_available: false });
+  backend.on('GET', 'config', { metadata_fields: {}, mode_labels: {}, library_available: false, download_fields: {} });
   backend.on('GET', 'jobs', []);
   backend.on('GET', 'stories', []);
   backend.on('GET', 'rules', { rules: [], paragraphs: false });