* stop using the ConfigContext and instead print all the public variables
  the Javascript may want from the server in a `window.formspree` variable
  (see formspree/static_pages/helpers.py).
* very ugly UI that gets address, country, name, zip with very basic validation
  plus Stripe Elements to get card details, send everything to stripe and get a token
  back, send that to /buy handler that should do something with that (it includes
  email and password in case the user needs to be registered).
* /cancel handler is called when the user is already a member of some plan and wants
  cancel. that server part is yet to be implemented too.