added remove_named_servers setting for jupyterhub-idle-culler

[stevejpurves]

If you are using TLJH with named servers, it can be a requirement that the server is removed when culled. This PR is to expose the underlying IdleCuller option, allowing it to be set via tljh-config.

• Add / update documentation
• Add tests

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[stevejpurves]

I noted some issues with the test suite while getting this to go green. It seems like he mock config can leak between tests meaning it was not possible to add a test for remove_named_servers independently of users.

Also I added a test case where potentially the existing test case for services could be extended instead -- let me know if I should change that.

[consideRatio]

This PR has merge conflicts that needs to be manually resolved, but this LGTM and can be merged when those are resolved!

Thank you @stevejpurves for your work on this!!! ❤️ 🎉 🌻