Fix Unauthorized exception in back references adapter

The problem is caused by PloneHotfix20130618.
Calling `getBackReferenceImpl` might raise an `Unauthorized`.

The fix is to switch to system security for the `getBackReferenceImpl` call, so that we also get objects where the current user has no access. This is by intention because also objects invisible to the current user should be respected.

/cc @maethu