FEATURE: Allow the plugin to be configured using site settings

GlobalSettings configured via environment variables will continue to take precendence, so this change is backwards compatible.

- `.presence` is added to string values, since 'unset' site settings are never `nil`
- saml_force_domains is split on `|` and `,` because site settings expect a `|`, while the old global setting expected a `,`
- Specs are migrated to use SiteSettings, because GlobalSettings do not override SiteSettings in test mode