adding preview mechanism.

As admin, add ?preview_as=user to the url to show the page as a regular user.
deactivating cached right management for local admins for the moment.

The local admins are not needed at the moment. Therefore, this is not killing a feature.

The problem is that the cache apparently is not flushed reliably.
For example, when switching the global_admin property, i.e. removing or adding a user group membership,
the cache is not flushed.