UX feature: heads-up for new users about minimum password length 

Actually it can - both `password_length` and `email_regexp` can be configured on a model basis, while the global configuration from the initializer is used as a fallback (Everything that is defined in a `Devise::Models.config` call like [this one](https://github.com/plataformatec/devise/blob/447b7030f856a3e1e9a08f3cc038977587420b9c/lib/devise/models/validatable.rb#L61-L63)).

```ruby
User.email_regexp
# /\A[^@\s]+@([^@\s]+\.)+[^@\s]+\z/
User.password_length
# 8..128
User.remember_for # config from the rememberable module
# 14 days
```