Make synchronization service extendable, update userprojects model

If you can not delete an Object from a database and then add it again then I see that as a problem of the database itself and not of the use case that uses it. Therefore this should be covered by a edb integration test, at least from my point of view. However, I can provide a test later on if you think it is required, but can you pls approve the PR for now so I can continue my work in the cdlflex project. Thank You!