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`.

pwplus/securedrop,jeann2013/securedrop,micahflee/securedrop,jaseg/securedrop,jrosco/securedrop,chadmiller/securedrop,conorsch/securedrop,ehartsuyker/securedrop,ageis/securedrop,chadmiller/securedrop,heartsucker/securedrop,garrettr/securedrop,kelcecil/securedrop,ageis/securedrop,pwplus/securedrop,jeann2013/securedrop,chadmiller/securedrop,heartsucker/securedrop,conorsch/securedrop,heartsucker/securedrop,chadmiller/securedrop,ehartsuyker/securedrop,jrosco/securedrop,pwplus/securedrop,kelcecil/securedrop,ageis/securedrop,conorsch/securedrop,heartsucker/securedrop,pwplus/securedrop,jrosco/securedrop,jaseg/securedrop,jeann2013/securedrop,ehartsuyker/securedrop,micahflee/securedrop,heartsucker/securedrop,kelcecil/securedrop,jeann2013/securedrop,micahflee/securedrop,chadmiller/securedrop,kelcecil/securedrop,pwplus/securedrop,pwplus/securedrop,garrettr/securedrop,jrosco/securedrop,ehartsuyker/securedrop,kelcecil/securedrop,micahflee/securedrop,ageis/securedrop,jaseg/securedrop,jaseg/securedrop,ehartsuyker/securedrop,jeann2013/securedrop,garrettr/securedrop,kelcecil/securedrop,ehartsuyker/securedrop,jaseg/securedrop,jaseg/securedrop,conorsch/securedrop,jeann2013/securedrop,jrosco/securedrop,garrettr/securedrop,chadmiller/securedrop,jrosco/securedrop,conorsch/securedrop