Constants should be centralized

[gunesmert]

We should be centralizing the constants that we are using throughout our codebase. My first suggestion for this could be for margins that we are using. For instance, I aim for sth like this:

enum Constants {
    enum Margin {
        static let extraSmall: CGFloat = 4
        static let small: CGFloat = 8
        static let medium: CGFloat = 16
        static let large: CGFloat = 24
        static let extraLarge: CGFloat = 32
    }
}

Here, using an an enum and static properties let use to reach them directly. Also, by using an enum, we are making sure that no one can instantiate Constants.