Allow use of new Django4.2 STORAGE config

[mpadjen]

Django 4.2 has introduced STORAGES setting that allows nicer management of multiple storages. It would be nice if this package allowed setting something like:

STORAGES = {
    "thumbnail_default": {
        "BACKEND": "easy_thumbnails.storage.ThumbnailFileSystemStorage",
    },
}

instead of only relying on "older style" THUMBNAIL_DEFAULT_STORAGE