Fix returned attribute logic

[meg-gutshall]

Currently the Loan class contains a boolean attribute called returned, however, it's occurred to me that a loan isn't returned, a loaned item is.

To fix this we can either:

1. Rename the returned attribute to active to indicate whether or not the loan is currently active, or
2. Remove the returned attribute from the Loan class altogether and add it to the Item class.

I'm more partial to the first option because if we go with the second option, we'll have to refactor other code as well.

[meg-gutshall]

Consider using ActiveRecord Enums for this refactor.