Use `gh` command line tool (if installed) to guess GitHub username

[fingolfin]

Basically, running gh api user --jq .login gives us the GitHub username. We can use this to provide a default for that question.

The user should still be allowed to override it, as they might want to use a different account or organization.

(I guess the question for GitHub username could also be made more precise, and make it clear that a GitHub org can also be specified)