Streamline the backend logging

Closes #880 (closed)

• Multiple commands now use Django's command output and error facilities, removing many loggers.
• All calls to logging.basicConfig have been removed, letting Django's settings alone handle the logging configuration without being overriden.
• A few loggers that were defined in modules but not used at all have been removed.
• The S3FileMixin logs for instances that do not have S3 objects (such as elements that should not have thumbnails) are now marked as debug to avoid polluting the production logs.
• The permission check failure logs are now marked as debug to avoid polluting the production logs (we already see HTTP 400/401/403 anyway).
• The Transkribus credentials error log has been removed (we already have a HTTP 400).
• The logging configuration now ensures that all django.*, arkindex.*, rq.* loggers are set to INFO.
• arkindex rqworker does show the logs for every job now!