support other key hosts beyond github

right now we use github because: https://github.com/maxogden.keys

but we should be able to use any scheme where you can type a username and it can figure out how to get a public key. it should probably prompt the user and double check that they wanna trust the public key from the domain it resolves, similar to what keybase does.

keybase might be a good next one to support actually, as they have a CORS enabled endpoint to get the pgp key for any user

Start by reviewing the existing GitHub public-key lookup described in the issue and how webcat authenticates users with those keys. Define the supported username-and-scheme flow and the trust confirmation; done means another host can provide a user's key only after the user approves the resolved domain, with Keybase considered as the next provider.

Written by the indexing model from the issue text.