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.

Ran unit tests.