Add donations to donation levels

A user can choose to donate at a specific donation level for a specific project. 
The donation amount, associated donation level, and donating user id are recorded in the database.
It's a placeholder until I add the balanced_payments. I needed to use a form in the projects page to collect and pass data to balanced_payments, so the simplest thing was to use create for the placeholder page, which is where the form's "post" tries to go by default.