[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
`ng-animate` adds its own magic that conflicts with the CSS animation of the spinner. Maybe there is a way to make them play nice together without excluding the `myspinner` class. I mean it would be nice to have nice fade-in / fade-out effects for the spinner. But for know I excluded the class, which I think works okay for the time being.