DIG-1178: New AuthX model

[justin-ys]

• Adds is_permissible(request, dataset) method to validate individual requests
• Changes get_opa_datasets to get_readable_datasets
• Deprecates is_site_admin (still used internally until OPA update is ready)
• Backwards compatible, but warnings are provided for deprecated behavior
• Changes/appends tests appropriately

[kcranston]

There was some discussion about refactoring the is_permissible method to accept either a python request object, or a json object containing the data. This would make it a non-breaking change for other services.

[justin-ys]

I think is_permissible won't accept a request object, since it's a new function anyway so nothing would be using it (except my branches, which have already moved to the new request model anyway). But this has been implemented for all other functions

[kcranston]

I am working on the OPA side now. We can't merge this until that's done, because the is_permissible stub is, well, too permissible and would make integration tests fail. Specifically, I don't want to have this merged into develop:

   elif request["method"] == 'GET':
        return True

Open to suggestions for a safer workaround that would let the rest of the data ingest project proceed.