Improve pghistory logging and admin page

[grossir]

We implemented the pghistory admin page for a single object on #4602 . There is room for improvement:

• excluding updates that only update the "citation_count" field (are there any other fields that make non interesting diffs?)
• ordering by creation date. For some reason it's not the default order, check this one
• try a slightly better formatting for pgh_diff. I think doing a json.dumps(pgh_diff, indent=4) will do the trick to make it nice to read