Allow identity proof for keybase

Keybase has opened their API for any platforms to do identity proofs:

• Blog post: https://keybase.io/docs/proof_integration_guide
• Documentation: https://keybase.io/blog/keybase-proofs-for-mastodon-and-everyone

This would add the possibility to proof that an identity on e.g. gitlab.com/leipert belongs to the same person as on Twitter and other platforms. e.g. See https://keybase.io/leipert:

It would be interesting to have this set up for gitlab.com. I don't know about private instances though.