Sorting contributor stats by username, not user id: fixes issue #367

Started over with a fresh fork and using branches this time to make edits (sorry, new user). I looked at issue #367 that requested contributors to be sorted by name in the stats tab of a project. I was able to narrow down the database call that retrieves the contributors for the stats page in osmtm/views/project.py. Based on the definitions from osmtm/models.py, I believe I am able to call User.username in the order_by command from the database query to sort by that field.