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

Sorry in advance...first time contributor. 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.

Please, comments and critiques are welcomed. I am still setting up a test environment on my machine, so this hasn't been tested yet.