IDP dynamic configuration

[challet]

This PR adds two main dynamic and optional configuration settings :

SAML_IDP_CONF_LOADER : a path to a callable that can adapt the IDP configuration, taking two arguments :

• static_config : the configuration dict defined by the existing SAML_IDP_CONF setting
• request : the http request being handled

SAML_IDP_FILTER_SP_QUERYSET : a path to a callable that should return the queryset of SPs available along the request being handled. It takes two arguments :

• original_queryset : the queryset orginaly used : all active SPs
• request : the http request being handled