Implemented guest checkout and shipping

This is a pretty major PR.  The primary change is the implementation of 'guest checkout' - which means that users can now contribute without having to create accounts.  Overall it makes the process more seamless and prepares the crowdhoster platform for some fun future capabilities...more on that soon :)

Also implemented is the option to collect shipping address from contributors.  Still need to add validation for these new fields and a way to extract the data, but the collection part is set up!

Lastly we cleaned up the checkout amount selection and payment screens a bit