- Add information about Neovim

This reverts commit e3aa55cc.
$PWD does not work when deploying as a Swarm stack.

- https://github.com/jupyterhub/jupyterhub/issues/4834

- Update README.md

- Update sample jupyterhub_config.py
- Update references to current state

- Remove `version`
- `version` is obsolete

- Update references to current state

- Pin PostgreSQL version

- Pass environment variables to jupyterhub

- Add callback URL

- Remove fosspay

- Add information about Git LFS
- Add link to reference deployment