Handle Payment Intent requiring additional actions

When a payment intent is created by the API (eg. the user added a card and paid for month 1 and now the API is attempting to charge them for the 2nd month) and this payment intent requires additional action from the user (Such as providing 3DS authorization) we need to notify the user that there is an outstanding action required, and allow the user to take this action.

An API endpoint to fetch these payment intents will be required, as well as some UI to display these (notification area). Clicking on the notification would bring up the URL specified in the `PaymentIntent.nextAction` in an iFrame allowing the user to perform any authorization steps required by their bank.

Inspect the API entry points for PaymentIntent records and the UI notification area, then trace how PaymentIntent.nextAction is exposed and rendered. Confirm the endpoint can find intents requiring user action, the notification opens the specified URL in an iframe, and the 3DS authorization flow can be completed.

Written by the indexing model from the issue text.