Support book URL and checking if user exists for GitLab

[joelostblom]

Minor suggestions, currently the book URL and user check is hardcoded to GitHub. Maybe there could be a choice in the beginning about which version control host to use (like with the end for the CI now). Then the rest of the options could be generic ("username", "ci", etc) and the host URL would be prefaced where needed.

Changing the url to gitlab also gives this warning so maybe binder should be turn off if gitlab is chosen (with a messge): WARNING: Currently Binder/JupyterHub repositories must be on GitHub, got https://gitlab.com/

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