allow using non-disk attachment storage

[patroqueeet]

because of docker, s3 and others: it'd be great if we can configure (and on change migrate) random Storage class to save the attachment files.

[blag]

Fixed in #489, I think.

Specify the storage backend of your choice in your Django project's settings.py:

STORAGES = {
    ...,
    "my_custom_post_office_storage_backend": {
        "BACKEND": 'storages.backends.s3boto3.S3Boto3Storage',  # Or whatever you prefer
        "OPTIONS": {
            ...
        },
    },
}

and then configure Post Office to use it:

POST_OFFICE = {
    ...,
    "FILE_STORAGE": "my_custom_post_office_storage_backend",
}

No database migrations are necessary.