Add configuration to limit permitted `format()` conversions

In order to limit the number of thumbnails generated (to save resources) it would be good to be able to limit the number of formats created by the format() filter. For example, to only allow WebP and PNG images to be created for PDFs and PNGs for SVGs:

ALLOWED_CONVERSIONS = {
    'pdf': {'png', 'webp'},
    'svg': {'png'},
}