This is still a little clunky and repetitive, code-wise, but a nicer
interface.

The `settings` script now has 3 subcommands:

    diff    Show differences between instance and settings.yaml
    update  Update settings on instance to match local settings.yaml.
    view    Show all instance settings.

Additionally, it takes parameters for instance URL and token, which can
also be specified by environment variables.

Background:

  - https://click.palletsprojects.com/en/8.0.x/
  - https://pypi.org/project/click/