add mechanism to block users

[ericphanson]

in General we have seen some spam AI generated registration requests from the same user. It would be great if Registrator could block users. General itself can't block users since the PRs come via registrator, so blocking a user from the repo doesn't block their registration requests.

This could work by making Registrator check a private github repo (with a configurable URL for other deployments) that has a list of users or something like that.