Concurrent edit of group EP not save

How to reproduce

• Create a group with user A and B as member
• Enable the EP tool for the group
• With user A, open the EP tool and create an entry. In the entry create some parts
• Login with a second browser with user B and open the EP tool in the group, open the same entry
• With user A, delete a part
• With user B, try to edit the part that has been deleted by user A
• => NOK RS

Expected behaviour: a message that tells the user that this part has been deleted by someone else. 

I did not test what happens when reordering things, if this messes up things as well, or adding some parts by both users at the same time. Ideally a lock would be made on a single part and moving parts around would be pushed via a multi-user event to all users who are collaborating on a page. 

Making it possible to concurrently edit a page would be a killer feature for the long-term usage of the editor as a collaborative page editor. However, for now this is not a mandatory requirement, we implement it only if feasible with acceptable amount of work. For the group EP a temporary solution could be to simply put a GUI lock to the entire page editor so that only one person at any time can edit a page.