Added endpoint for the 'site' JSON (In the preload store)

Added endpoint for the 'site' JSON, as per [Robin's (EvilTrout) suggestion](https://github.com/discourse/discourse/pull/1949) to [my issue](https://meta.discourse.org/t/subcategories-in-categories-json/11445/10).

All this is doing is making an endpoint for the 'site' json that is found in the preload store.