Until now, Astakos exposed statistics per authentication provider.
However, since a user can have more than one authentication providers,
the administrator cannot sum the values per provider in order to compute
the total values. This commit fixes this issue by extending the Astakos
admin API to expose statistics for all users and resources.

Also, this commit updates the resources per provider response, to
include only the resources that are allocated to users that are using
only this provider.