Added in domain items for a MediaItem and a BorrowRequest, fixed foreign keys between MediaItems, ApplicationUsers, and BorrowRequests. No way to interface with creating Media Items yet, but I added a test seed to show that it is created in table. Note: to do this I had to give tester an Id, which is obviously not what we would do, but that goes along with the authentication part in retrieving a user's Id, which is not finished yet. Feel free to remove that line from the seed when it is done.