switch to nix flakes

[wd15]

Update PFHub to use Nix Flakes rather than the old shell.nix protocol. Flakes are essentially far more composable than the shell.nix way.

Changes:

• Single flake.nix environment for running everything. No more separate environments. There is a flake.nix in _data/python-pfhub for development of the Python package, but the flake.nix in the base directory references that.
• All *.nix files are now gone and replaced with only twp flake.nix files
• No longer using htmlproofer as building it with Ruby was presented too many problems. Not so concerned about this, but we're no longer testing links.
• Stopped using Travis CI. Functionality moved to .github/workflows/test-jekyll.yml.
• Updated Docker to use flakes
• Removed _apps as the apps are no longer used
• Cleaned up code in _data/python-pfhub to account for new versions of the linters
• new docker image on docker.io
• update documentation for nix and docker changes
• nix generates the docker image now