Fix InvalidPasswordLength per @conorsch's review
Fix InvalidPasswordLength per @conorsch's review

* Only store password length instead of the password itself, just to be
  safe, because the password itself might be sensitive (for example if
  it was a typo of a real user's password instead of a brute force
  attempt).
* Fix forgetting to get the attribute from `self`.

mark-in/securedrop-app-code,mark-in/securedrop-app-code,mark-in/securedrop-app-code,mark-in/securedrop-app-code