`use_github()` - allow repo creation when repo is a redirect

[ateucher]

This changes check_no_github_repo() to check the name of the response from the GitHub API against the provided spec (user/repo) - If they are different it assumes it's a redirect and then proceeds and allows repo creation. It retains the behaviour where if request to the GitHub API returns a 404 it similarly passes and allows the creation of the new repo.

Unfortunately the response returned from the GitHub API doesn't appear to explicitly say it's been redirected (i.e., it's a 200 HTTP code, not a 301).

Fixes #1893.

I didn't add any tests - though could add to the manual tests if you like?