fix #516: thumbnail_cleanup command for S3 and different source storages

[PetrDlouhy]

I was able to make partial fix for #516.

It enables thumbnail_cleanup to work correctly with S3 stoarges and works with different source and destination storage.

The default source storage is now set to DEFAULT_FILE_STORAGE (which is probably better choice in case when the source and destination storages differ), and can be changed by the management command --source-storage parameter. It also adds check for the source storage hash, and if the hashes differ, it only prints warning, but take no action.

May be we could detect all possible storages and compare their hashes with the source.storage_hash value. But I am not sure, if I can load all storage options from settings. The situation would be much easier, if the storage classpath string is stored directly in the Source objects.

Also this change doesn't cove cases, when user wants to change the source storages. In such cases it might be useful either to change the source hash or to delete the thumbnail reference based on users requirements.