Support alternative GitHub host for GitHub CI/CD auth flow

Proposal

We support a simple setup flow for connecting an account to GitHub via a Personal Access Token, but this flow doesn't currently work for GitHub Enterprise installations.

We should investigate adding a custom host option to https://gitlab.com/import/github/new?ci_cd_only=true to allow configuring this feature for a self-hosted Enterprise install.

This may be a simple change or more complex depending on how hard-coded the URL is in the setup, so this will require an initial investigation in order to weight the issue.