Access Control: fix workspace permission (#592).

The READ permission for Workspace topics is calculated properly.

Crucial refactoring: the logic to calculate the permissions has moved from the Access Control module to the Core module. This is required because the permission calculation must involve low-level Core calls (direct storage access) in order to avoid endless recursions. E.g. calculating the READ permission for a workspace topic must be performed without actual fetching the workspace topic itself (through the public Core Service) as this would involve calculating the READ permission...

BREAKING CHANGE

1 class moves to another package:
{{{
de.deepamehta.plugins.accesscontrol.model.Operation
-->
de.deepamehta.core.service.accesscontrol.Operation
}}}

See #592.