Remove the "safety mode" and default to using the node access layer.

This issue will focus on backporting the node access code from the D8 version of the module.

Even though the node access layer is far from perfect, trying to work around it in Group 7 was a bad idea in hindsight. It disallowed Group 7 to work well with other node access modules and introduced a system which was hard for others to understand. I will remove this in favor of core's node access system.

The node access code is thoroughly tested by PHPUnit in Group 8, so backporting it (almost) 1:1 should come without too many bugs.