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/apptask.js

@@ -689,6 +689,7 @@ async function stop(app, args, progressCallback) {
     assert.strictEqual(typeof progressCallback, 'function');
 
     await progressCallback({ percent: 20, message: 'Stopping container' });
+    await reverseProxy.unconfigureApp(app); // removing nginx configs also means that we can auto-cleanup old certs since they are not referenced
     await docker.stopContainers(app.id);
 
     await progressCallback({ percent: 50, message: 'Stopping app services' });