webview: Disable insecure content.

Zulip servers in production are designed to only serve content over
HTTPS.  And a development environment's root page will be served over
HTTP.

So there is no purpose in enabling allowInsecureContent, even
conditionally for use against Zulip development environments; we should
just remove the setting.

app/renderer/js/components/webview.ts

@@ -50,7 +50,7 @@ export default class WebView extends BaseComponent {
 					${this.props.preload ? 'preload="js/preload.js"' : ''}
 					partition="persist:webviewsession"
 					name="${this.props.name}"
-					webpreferences="allowRunningInsecureContent, javascript=yes">
+					webpreferences="javascript=yes">
 				</webview>`;
 	}