Provide `setup-github-mirror.sh` as tool to automatically set up GitLab
projects to be used for `mirror.sh` to keep upstream work around
locally.

A basic README should provide enough information for people to setup the
project successfully in their environment.

What `./setup-github-mirror.sh` does:
 - Collect information from GitHub about the repository to mirror
 - Create a subgroup in the `github-mirror` group with the github
username of the repository owner
 - Create the project with the github repository name within the created
subgroup
 - Configure the project to use the github avatar as project avatar,
disable issues, merge requests, CI/CD integration, wiki, and snippets,
and cofigure the project description to the repository description from
GitHub along with a reference to the upstream repository
 - Enable the deployment key of the mirror bot for the project and grant
it write access
 - Provide a ready to use `./mirror.sh` instructions

The goal of ...