Change Devise.confirm_within to Devise.allow_unconfirmed_access_for

According to Devise's changelog
(https://github.com/plataformatec/devise/blob/master/CHANGELOG.md),
Devise.confirm_within was deprecated in favor of
Devise.allow_unconfirmed_access_for in version 2.0.0.
Merge pull request #6238 from athos-ribeiro/devise_unconfirmed_access

Change Devise.confirm_within to Devise.allow_unconfirmed_access_for