E-Commerce

### Goals
The goal would be to provide a simple e-commerce experience that works out-of-box but can be easily customized by the developer. This feature would primarly be used for smaller clients who need custom business logic that would be difficult to program with online paltforms like Shopify. We seek to have a simple system that would be a suitable (and better) replacement for something like Woocommerce + WordPress.

Target website would be small business e-commerce, or digital goods. Large retailers dealing in high volume would probably defer to a more specialized system.

### Basic requirements
* Product management/pages in wagtail admin.
* Product category management/pages in wagtail admin.
* Support subscription products and recurring billing/auto-pay. Either through the payment gateway or something like a simple cron job or management command.
* Cart functionality (can be turned on or off if not needed)
* Payment gateway (I am fine with supporting Stripe exclusively at first).
* Confirmation and payment error email management in wagtail admin.
* Super basic order management - in the form of an order summary email for customers, and order status/notes in wagtail admin. No customer login/dashboard is required at this time, but that would be a need in the future.
* Super basic shipping calculations. We could look to other ecommerce systems for inspiration here.
* Basic tax calculations. Tax laws are complex, so we could support a flat tax-rate out of the box, and provide a hook for doing any more complex calculations. That way developers can easily program this or integrate with a 3rd party tax calculator service based on each individual website's needs.

### Implementation
Most obvious solution would be to integrate something like [Longclaw](https://github.com/JamesRamm/longclaw) or [Oscar](https://github.com/django-oscar/django-oscar). I'm not the biggest fan of Oscar although it is extremely robust. Longclaw looks appealing - perhaps we could join forces with longclaw team and contribute upstream, or fork the project to ensure we can provide professional support and timely code changes as needed. Open to other Django-based solutions as well.

I envision this being either a separate pip package, or an off-by-default feature. If many large dependencies are required, then a separate pip package would probably be more ideal.