This MR adds support for Hagrid to act as a WKD gateway.
The idea is that mail hosts can point a CNAME record on their
openpgpkey subdomain to
wkd.keys.openpgp.org. Hagrid serves keys for verified addresses under
.well-known/openpgpkey/DOMAIN/hu/ZBASE32(SHA1(LOCALPART)) , in keeping with the WKD "advanced" method.