Check for password strength on back-end

While we'll be implementing captcha support with a check if the password contains a blacklist item, we also want to check it on the back-end.

### Reproduction Steps

We check for the password strength requirements on the front-end, but a weak password (ie. the password being equal to one of the other input fields in the form) will succeed when passed directly to the back-end.

### Expected Behavior

Form submission is rejected due to password containing a blacklist item.

### Actual Behavior

Form submissions is allowed.