check if old connections are removed from DB (WIP)

Add new test which will create a flow, store it in DB, load from DB,
change a connection, then verify the original connection is completely
removed from all tables in the DB. The last part is not finished yet. We
have verified that the original connection is not linked to the flow
anymore, but it might still be in the DB.
In Flow.getElement() we now raise ElementNotFoundError instead of
returning None