Allow editors to clean up their own messes

One good improvement on the present state would be allowing users to merge nodes they have created themselves (and where no other person has been involved yet), without any supervision -- since it's entirely their fault (and perhaps the user interface they at first did not understand), it's only right they get the bits to wipe up the mess they made, if they want to. Adding a manual moderation step there just makes it cumbersome to fix errors.

I base this solely on having myself first added some members to the music ensemble "Visa Röster", then added some photos and marked up those photos with the people featured in them -- and, at that stage, made name duplicates rather than picking the "Member of Visa Röster" person from the pop-down menu. After realizing the error, I tried sweeping up some of the messes by marking some of those duplicates as duplicates, but quickly lost track (since those I had "fixed" were still left in the system), and stopped, since I did not remember which ones were left to do, and quite simply did not have the time.