[Sprint: 39] [master + backport] XD-2332 Provide Server-Side Cron Expression Validation

* Add validation REST endpoint
* Add AngularJS directive to perform async validation

Jira: https://jira.spring.io/browse/XD-2332
I actually set `ng-model-options` with `{ debounce : { 'default' : 1000, blur : 0 } }`. This means that only after 1 second an HTTP requests is performed while being in the form-field. Once you leave the form field (blur), the validation is executed immediately.