Refactor logging mechanism.

[philippeowagner]

Use Django's built in logging system instead of the one implemented in the management command.

[jvamvas]

@philippeowagner Implemented in the current master branch:

• 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. The django LOGGING setting can be used instead

[walterrenner]

@jvamvas this is resolved in 1.1.0rc2, right?

[jvamvas]

@walterrenner Right.