Configurable maximum uploaded file size

[psrok1]

We already have some limitation involved, but it's just hardcoded in one of nginx configurations:

    client_max_body_size 50M;

https://github.com/CERT-Polska/malwarecage/blob/master/malwarefront/default.conf.template#L6

Moved from CERT.pl internal repository. Reported originally by psrok1