Work in progress on checkout: validate and create user
Added stub for posting payment info to perusion
Added method for redirection url
Fixed test number
Use update_or_create on post.

There is no guarantees that the user is really who it's saying it is.
Added redirection and payment order entry (work in progress)
Added class to handle the postback
Added postback route for silent post

Currently it's not handling faulty request correctly.
Added ReverseProxy middleware to check the source of the postback
Do not die without reason. Instead return and log the error
Pass the transaction id (rid) to Perusion
Add style for validation errors.