[BUGFIX] Allow explicit privileges for ``Everybody`` role
The ``TYPO3.Flow:Everybody`` role that is always active by design gets
an ``ABSTAIN`` permission for all defined ``PrivilegeTargets`` so that
permission to restricted privileges is not granted by default.
Due to a bug this ``ABSTAIN`` permission was also set when a privilege
was explicitly configured for the ``Everybody`` role as soon as the
same ``PrivilegeTarget`` was used for another role, effectively
overriding the permission.
This change allows for explicitly allowing or denying privileges to the
``Everybody`` role. It also adds some unit tests for the
Releases: master, 3.0