Allow to configure a default Merge user

Right now forcing to use change requests involve to deny the edit right. However the edit right is needed for merging them. So it can problematic if the same people who edits through CR should also be able to merge.
In order to solve this situation, we should be able to configure a default Merge user: by default this configuration would be empty, so that the current user would be the merger. But when filled with an actual user having proper rights, this user would be always used to perform the merge.