The python3-saml library has a bunch of settings that govern its behavior. We have siteconfig keys for the settings that matter (things like binding type, verification keys, URLs, etc), which then gets turned into the dict format that python3-saml wants. In some cases, users may need to override those settings. In a recent report, Windows machines which use x509 to log in require setting some items in the `security` key (which we otherwise don't use at all). This change adds an escape hatch for those situations where admins can define `SAML_CONFIG_ADVANCED` in their `settings_local.py` file, mimicking the advanced_settings.json file that python3-saml would use if we were not passing in the config as a dict. Testing Done: Ran unit tests.