Validate payment forms using javascript so we don't create multiple uncaptured paymentIntents when the form is not valid

Fix repeatContribution - pass the found contribution ID instead of using a separate previous_contribution variable - fixes \!2

Support recurring payments with paymentIntents/Elements. Cancel subscription with Stripe when we reach recurring end date

Handle confirmation pages properly for contribution pages (make sure we pass through paymentIntentID

Cards that are confirmed by the user (eg. via 3dsecure) require confirmation before the payment can be captured. Other cards don't. Support both

Switch all instances of setAPIKey to use shared function so we ensure that API version and info is always set