Port Django's CommonPasswordValidator to Rails + Devise

[eoinkelly]

Django includes a password validator which validates passwords against a built-in list of 20k common passwords. I think it would be neat port this to Rails + devise.

The source code for the devise Validatable module demonstrates how we could implement our own custom validations. It may make sense to eventually package this as a gem but we can cross that bridge if/when we have a working version as a PR here.

Another option worth considering is the devise_security_extension gem.