Preview checkout #17
Description
It would be very useful to be able to "preview" a checkout by rendering its templates against a mock Actionkit context.
This is kind of a big task -- it involves reimplementing enough of Actionkit's custom template directives to actually render the templates. And then building a "fake context" management system that lets users set up local fake pages with -- at minimum -- types, custom pagefields, and perhaps actions already taken.
I started looking into this and got hung up on the {% include %} feature -- I think we'd need to somehow implement a custom Django template loader which is aware of the current context, so that {% include "user_form.html" %} includes the right codeblock from the right checkout.
(Alternatively we could just deploy the checkout to a new templateset, clone a particular page, and preview it live in Actionkit. But that's less fun!)