This commit adds ability to fetch a subset of site settings from the `/admin/site_settings` endpoint so that it can be used in all places where the client app needs access to a subset of the site settings.
Additionally, this commit also introduces a new service class called `UpdateSiteSetting` that encapsulates all the logic that surrounds updating a site setting so that it can be used to update site setting(s) anywhere in the backend. This service comes in handy with, for example, the controller for the flags admin config area which may need to update some site settings related to flags.
Internal topic: t/130713.
