This PR moves the logic from the setting-object mixin to a helper class. I've opted to maintain the interface of the previous classes (ThemeSettings / SiteSetting) that used the mixed-in methods through aliases so that we limit the amount of changes here (these are referenced in the string form and across classes/templates).
Another option we may consider in future if we want to optimize for performance is straight duplication which would trade off this overhead of aliasing/chaining calls through the helper for some duplicate code - only 2 models require these methods at the time of this PR.
