Djangosaml2-tenant

- Removed the logic of loading metadata from database model tenant.Member. Projects using this app can specify
their own config loader using the SAML_CONFIG_LOADER setting. This change provides more flexibility in-terms
of loading config from any data-source.
- Also, removed the caching of service provider config objects as per the same logic as above. Individual
projects using this app can use their own config loader which handles caching scenarios.

- Make djangosaml2 tenant support reading metadata from database by adding the following to settings.py:
SAML_CONFIG.update({
Specifying 'inline': 'DB' means SAML metadata comes from the DB via the following method:
tenant/models/Member.get_saml_metadata(self)
Implement this method to retrieve the metadata from the table of your choice.
'metadata': {
'inline': 'DB',
},
...
})

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- Initial version based on djangosaml2 supporting multi-tenant