Remove Realm and ObjectMapper Dependencies
Summary
Realm is a third party Database that, frankly, we don't really need to be using. Core Data is a built-in database solution to iOS, and it would be nice to transition to this in order to reduce our dependency load and to be using more of the system's provided APIs.
ObjectMapper turns our downloaded JSON into Swift classes. This has been shellacked by Codable in Swift. We should switch to using Codable in order to better use Swift's features.