Updates to Subscription#index view page

This pull request fixes the display of subscriptions for guest users, as it was calling id on nil before.

It also includes a new Subscription scope `post_cart` that filters out cart subscriptions for the `Subscription#index` view because they're not valid subscriptions yet. It updates the `SubscriptionsController#index` action to use the new scope.

And lastly, it updates the `SubscriptionsController` to use `BaseController` which includes authentication checks. Before this change you could view the subscription index without being logged in as an admin.