Changes in https://github.com/chrismedrela/django/commit/8779432394f42b05c83eb9c0a92d8f8fe941a265 :
* All issues are sorted by level and then by its `__str__` result (that means that they are effectively sorted by object name, because `__str__` method returns sth like `"object_name: message\n\tHINT: hint"`).
* The summary is: `System check identified [no problems|1 issue|%s issues].`
* (Not connected with this diff) Changed definition of `Error`, `Warning` etc. -- no need for intermediary `ConcreteCheckMessage`. I've also decided to use `functools.partial` instead of class definition.