Django-background-tasks

* Improved overall performance and stability
* Fixed DB race conditions during multi threading
* Improved admin site by adding the following attributes to list_display
* has_error: shows whether an error occurred when running a task
* locked_by
* locked_by_pid_running: shows whether the locked_by PID is sill running
* Improved thread pool scheduling

- add related_name to Tasks. creator_content_type ForeignKey

- Standardize logging:
- Raise log level of task failures from `WARNING` to `ERROR`
- Use logging as recommended in https://docs.djangoproject.com/en/stable/topics/logging/
- Remove `log-file` and `log-level` options from management command `process_tasks`. Please use the django [`LOGGING`](https://docs.djangoproject.com/en/stable/topics/logging/#configuring-logging) setting instead

Model changes (please make migrations and migrate when updating):
- Add `verbose_name`, `creator`, `repeat`, and `repeat_until` fields to `Task` and `CompletedTask`

Additional changes:
- Implement Task repetition; see https://github.com/arteria/django-background-tasks#repeating-tasks
- Add priority parameter to decorator
- Extend `Task` and `CompletedTask` managers
- The current LTS version of django (1.8.*) is now also tested with the `BACKGROUND_TASK_RUN_ASYNC = True` setting.

- Fix management command options `--log-file` and `--log-std`

- Legacy "autodiscover" function: Ignore apps that are installed via `AppConfig` path
- Avoid `UnicodeDecodeError` in setup.py for some locales