commenting: show CAPTCHA to new accounts (#443)

Basically re-use the upload CAPTCHA code to also do this for
comments.

nyaa/forms.py

@@ -205,6 +205,8 @@ class CommentForm(FlaskForm):
         DataRequired(message='Comment must not be empty.')
     ])
 
+    recaptcha = RecaptchaField(validators=[upload_recaptcha_validator_shim])
+
 
 class InlineButtonWidget(object):
     """