More logging

This hits four more application logs:

* The login form logs both successful and failed login attempts (along with the username). This was accomplished by extending the form's validation step
* Creation, edit, and deletion events through the admin interface are logged.The log includes the editing user's username, email address, the edited content's type, string representation, and id. This is accomplished via a signal listening for LogEntry creation
* User creation and deletion events are explicitly logged. Though this duplicates some of the above, this will also account for users created through manage.py. This is implemented through signals on the User model
* Media uploads and deletions are also logged. This is accomplished through a proxy file storage backend.