WIP start migrating to AmazonFlexPay gem
WIP start refactoring amazon lib
WIP Use AmazonFlexPay for endpoints
Use AmazonFlexPay to redirect to Amazon's CBUI
Replace Amazon::FPS::MultiTokenRequest with FlexPay
Use AmazonFlexPay to verify amazon's responses
Don't store objects in the session
Clean up Contribution Controller
Remove Contribution status `retry_cancel`

It really doesn't matter if we cancel it or not, and the admin can take
care of the individual cases better than the code. Automatically
retrying just causes a lot of unneeded complexity.
Don't retry on failed payments.
WIP more amazon stuff
WIP more amazon stuff