Moving all Rails app files and folders into backend folder.

As I planned in the [Project Code Structure](https://docs.google.com/document/d/1UmSkTYFu2ziLknzVMI_VfUhr3rskznMa2O4VszD26ZM/edit#heading=h.kfqora5g7mee) document, I moved all Rails app into backend folder. 

@amrnt can you review this and :+1: it .