Add locale and other environment variables support for systemd service jupyterhub

[wqh17101]

• Add / update documentation
• Add tests
  no more tests are needed.

#410

[wqh17101]

@yuvipanda @GeorgianaElena @minrk please review

[consideRatio]

I understand that this PR is centered about the technical change of appending the locale as reported via os.popen("locale") to environment_for_system_service.conf, and then making the jupyterhub registered service run with that file.

I think the idea sounds good, but I'm quite uncertain on how the LANG environment variable functions. Perhaps the feature can be documented more general about setting env vars for the jupyterhub service process and then providing one example that the LANG environment variable for example can be configured via this feature.

I added a lot of small syntax details to the documentation which I also think could be revised to be a bit briefer.