Clean Orphans from S3 Bucket

[noliveleger]

New management command: remove_s3_orphans
What it does:

• List all files from the bucket AWS_STORAGE_BUCKET_NAME
• Try to find a match in tables logger_attachments, view_exports, kpi_export_tasks based on the filename pattern.

Rules:

• If filename does not match required patterns, continue
• If filename does match one of the required patterns and it exists in the DB, continue
• If filename does match one of the required patters and it doesn't exist in DB, delete

Because of LifeCycle rules on our buckets. The file will be mark as a deleted will be kept for another 30 days in previous versions. We should find a way to tell DjangoStorage to permanently delete the file (or add a tag to it, and set another rule on the bucket to delete those files after 1 day).
Edit: All versions are deleted now.

This management command covers one kpi table. I think we should add kpi_imports too and create another management command in kpi which does the same with its own table.

Closes kobotoolbox/tasks#301

[noliveleger]

Closed in favor of #579