REFACTOR: Move further away from `Discourse.SiteSettings` (PR #10234)

This PR automatically injects settings in to all RestModel instances regardless of whether they are created via store or via calling create() directly, so that they can use this.siteSettings instead of Discourse.SiteSettings.

This exposed some other bugs where our test environment could have multiple instances of Discourse.SiteSettings, so this is changed to rely on a single instance which should cause less leakage between tests.


These old ways of achieving the same thing are no longer necessary.

Current User relies on Site Settings so it must be created first.