[major] API PGP authentication

    This is a major change to the client code and the API. All API calls now require
    a HTTP header called X-PGPAUTHORIZATION that contains a signature from a
    known investigator. This work is also the opportunity to split up the API client
    part of the MIG console into a new package called mig/client. This will facilitate
    code reuse, particularly for standalone command line utilities.