Emit a 'site_settings_loaded' signal when loading siteconfig.
Extensions may need to override loaded settings, such as adding

additional auth backends. In order to do that, it needs to ensure it can

operate on the settings after loading from siteconfig. This can now

be done through a new signal, 'site_settings_loaded'.

Used this in an extension, and was able to reliably update settings after

siteconfig loaded.