Add signature support for image resize operations

[Jotschi]

It should be possible to use a signature to sign image resize requests in order to prevent DoS attacks on the image API.

Examples:

• https://assets.orf.at/mims/2018/48/06/crops/w=171,q=90,r=2/73449_1k_13606_gen_babies_ap.jpg?s=a020a9aa42cf1e49754fc45212df00b03b25555f