Validate issue input in epic before clicking Add
In #4000 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
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).