Signup form username uniqueness checked only after captcha solved and form submitted

[benplumley]

On https://exercism.org/users/sign_up the username field's uniqueness isn't checked until after the captcha has been solved, and the form submitted. If a non-unique username was chosen, the form fails with an error, and the captcha solution is cleared. This means the process of creating a new account involves completing a new captcha for every single username attempted. After trying three usernames (solving six pages of captchas), all of which had already been taken, I've given up trying!