Referenceprefix manager fix, when subitems are already removed.

When a repositoryfolder get's removed by a user the reference_prefix manager for the parent object fails with an KeyError because the intid utility can't get the object (see [current implementation](https://github.com/4teamwork/opengever.core/blob/fa8dff2032e2624b8ad6c2cf97f2f8e38ae3db3a/opengever/base/adapters.py#L169))

This PR solve this problem, additionally i've reworked the tests (including ftw.testbrowser) and fixed another bug in the unlock link generation.

Following a simple example how the prefix manager looks with deleted objects:
![bildschirmfoto 2014-09-04 um 08 45 41](https://cloud.githubusercontent.com/assets/485755/4145764/5093dee2-33ff-11e4-9792-2a4c6a531793.png)
@lukasgraf please take a look ...