Show minimum password length on edit password

Shows the minimum password length for passwords/edit like it is already shown for registrations/new.

The following code is now repeated a total of three times (twice in `RegistrationsController` and now also in `PasswordsController`.) Is there a better place for it to go?

```ruby
@validatable = devise_mapping.validatable?
if @validatable
  @minimum_password_length = resource_class.password_length.min
end
```