Admin views payment feature.

- Added stripe_event gem to track stripe payment events.
- Mounting of StripeEvent::Engine to allow for post requests from Stripe.
- config/initializers/stripe_wrapper.rb updated for stripe_event.
- Testing added for /stripe_event route.
- Feature test for admin_views_payments.
- Route for admin_payment_path index action. Controller added as well.
- Payment table created. For keeping track of payment events sent from Stripe.
- stripe_id column added to users table to keep track of Stripe customer id for payments.
- Payment Fabricator created for use in admin views payment feature test.
- Functionality in user registration feature test moved as macro to be shared with admin views payment feature test.
- Tests added where required for additional Strip id requirements for user and payments.