roll settings commands into a single wrapper using click

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.

TODO:

• make target instance configurable

Background:

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