add support for GitLab instead of GitHub

[davewhiteland]

Thanks for the 2023–24 RHUL Foundation cohort running on local GitLab this time, we need to allow alternatives to GitHub:

Maybe need name and URL for the service which by default is "GitHub" and https://github.com — but might not need to URL since it's already in BUGGY_EDITOR_GITHUB_URL.

Should this rename the existing config settings? BUGGY_EDITOR_VCS_URL instead of BUGGY_EDITOR_GITHUB_URL — refactoring will break any existing implementations, but I don't believe there are any.