Enabling Customization of Storage Backends

[zachbellay]

Overview

This PR enables customization of storage backends. It introduces a new setting IMPORT_EXPORT_CELERY_STORAGE, and can be used as follows:

# myapp/settings.py
...
IMPORT_EXPORT_CELERY_STORAGE = "storages.backends.s3boto3.S3Boto3Storage"
...

By setting this field in your django settings, you will be able to customize where all files related to django-import-export-celery go.

In my case, I would like these files to have more access control, so I would do something like this:

IMPORT_EXPORT_CELERY_STORAGE = 'utility.s3storages.PrivateMediaStorage'

Rationale

This PR is necessary because the only way to customize the storage backend is by modifying the default_storage setting in django. However, since this applies globally, if you were to set default_storage to a private/protected storage backend, you would be unable to have publicly accessible media, hence this request for change.

[timthelion]

This works for now, but I've run into the exact thing you're facing and TBH I think we should modify Django to be able to set a flag as to whether a FileField should be private or not.