Introduce new document state 'removed'

Add new permission `Remove GEVER content`, including transitions for `removing` and `restoring`.
To protect the removing transition separately, I've also add a new Permission `Remove GEVER content`.


This PR solves Issue #657

@lukasgraf please have a look ... 
This currently does *not* test that documents can **only** be removed with the permission. It would be nice to have a negative test case, where you use the `Contributor` role and make an assertion on the transition not being allowed.