Reserve my spot

https://trello.com/c/YsIjoogQ/147-setup-simple-student-reserve-my-spot

When a student application is approved, if the student application has a "Will pay registration fee" checked (this checkbox needs to be created in SF and in Rails), the student will receive an email (yet to be created) from Salesforce directing them to log into the Rails site.

Once in the Rails site, they will click "Reserve My Spot"

This will charge them $25 USD through Braintree and once completed will allow them to see the student profile page

-SF Callback to Rails when Student App is "Approved"
-Sync whether student app needs to have a payment or not
-Show Student payment form when the "Reserve My Spot"
-Enable student profile once student has paid

This branch already has merged #58 and #62. 
el student_id no es obligatorio, por ejemplo el caso donde se dona a un proyecto directamente. Según entiendo son 3 tipos de donación: a converge, a un student_application y los 25 de reservar lugar, entonces dependiendo del tipo el método create recibe ningún parámetro, el student_id o el student_application_id.