WebView: Enable allowpopups.

This is required for Electron ≥ 15 to continue invoking our new window
handler (handleExternalLink), following the nativeWindowOpen
migration.

https://github.com/electron/electron/issues/30886

Signed-off-by: Anders Kaseorg <anders@zulip.com>

app/renderer/js/components/webview.ts

@@ -71,6 +71,7 @@ export default class WebView {
           ? html``
           : html`preload="${props.preload}"`}
         partition="persist:webviewsession"
+        allowpopups
       >
       </webview>
     `;