Validate issue input in epic before clicking Add

In #4000 (closed) we're validating errors right after we click Add in an epic. However, to have the best experience, we should start validations even before the user clicks Add.

Also, some things can be easily validated on the client-side with no need of server logic, like the URL domain and the group/project path (e.g. if the domain is different from the current page, it's immediately invalid).