Add field to skip backup for an app

This skips the app from a backup when doing a full box backup and
simply reuses the previous backup.

The app can still be explicitly backed up using 'cloudron backup'
and explicitly restored using 'cloudron restore --backup'.

When restoring the box, it all depends on the app's last backup.

Fixes #311

webadmin/src/js/client.js

@@ -338,7 +338,8 @@ angular.module('Application').service('Client', ['$http', 'md5', 'Notification',
             memoryLimit: config.memoryLimit,
             altDomain: config.altDomain || null,
             xFrameOptions: config.xFrameOptions,
-            robotsTxt: config.robotsTxt || null
+            robotsTxt: config.robotsTxt || null,
+            enableBackup: config.enableBackup
         };
 
         post('/api/v1/apps/' + id + '/configure', data).success(function (data, status) {