This involves a little refactoring of how our `defaultHomepage()` works.
It previously would check the meta tag / site settings each time it was
called but now it only checks once on application boot.
* Remove unused Discourse.SiteSettings
* Remove `Discourse.SiteSettings` from many tests
* REFACTOR: `lib:formatter` was using a lot of leaky state
* Remove more `Discourse.SiteSettings` from tests
* More SiteSettings removed from tests