Stop providing a default SKS keyserver
I propose to not provide a default config setting for Conf.sks_keyserver
anymore.
The old SKS keyservers are mostly dead, keyserver.ubuntu.com
appears to be the only public SKS keyserver left. We shouldn't encourage to use them anymore.
But keys.mailvelope.com
(validates uploaded email addresses) provides its keys via SKS-like URLs (besides its own API, which I don't want to implement), and people might want to use non-public SKS keyservers, too. Therefore I do not propose to remove handling SKS-keyservers completely.