Yearly billing cycles

As discussed, this allows users to subscribe annually when signing up and switch to monthly or yearly billing after they've already signed up. Stripe handles prorating subscriptions which makes things easy for us. One note about that: Stripe will issue refunds when a yearly subscription is canceled or changed to monthly before the end of the year.

I added poltergeist back into the mix so we could test the UI elements of changing billing. @sandal I know you had problems with that so let me know if we need to remove it from the test stack. There are no tests for stripe but I have manually tested all code paths and everything appears to be working as expected.