remove nginx config of stopped apps

when the cert of a stopped app gets auto-cleaned up, nginx does not
start anymore since the config references the cert.

there are two possible fixes:
* do not cleanup cert of stopped apps
* remove the nginx config of stopped apps

this implements the second approach

src/reverseproxy.js

@@ -672,6 +672,7 @@ async function cleanupCerts() {
             const fqdn = certFilename.replace(/\.cert$/, '');
             debug(`cleanupCerts: deleting certs of ${fqdn}`);
 
+            // it is safe to delete the certs of stopped apps because their nginx configs are removed
             safe.fs.unlinkSync(certFilePath);
             safe.fs.unlinkSync(path.join(paths.NGINX_CERT_DIR, `${fqdn}.key`));
             safe.fs.unlinkSync(path.join(paths.NGINX_CERT_DIR, `${fqdn}.csr`));