Increase max length of simple_captcha to 12

In response to issue #5157 i increased the max length of the captcha value field to 12, i also added a check to make sure the value is within the valid range
If it exceeds the maximum set it to the maximum maybe? Or just raise and kill the app.

For the set to max route: `sc.length = [1, [AppConfig.settings.captcha_length.to_i, 12].min].max`