• Utilize an enum for UserDefaults keys. We are aiming for a single location in the codebase to see an overview of what we are saving to UserDefaults.
• Clean out any we don't need anymore.
• Move to a unified key format. If you are changing old keys to the new format this will require migration.
• Note we also have the WMFKeyValue Core Data object. We should understand & document where one should be used over the other.

Note: We already have a helper extension at NSUserDefaults+WMFExtensions.swift. We should pass all usages through here.