User story: "As a user, I'd like to know whether the username I have chosen is valid before I try to create it, so that I can fix it first."
Basically, in the app the user has to continually resubmit requests before they know whether their username is valid or not. It'd be nice if, after they've typed in their username, we could send a request to the API to see whether it's valid or not, while they continue with the rest of the form. That way, we can inform them to change it before they submit.
Right now it's only possible to validate the username by trying to create it.