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/

This change matches existing settings, and is thus a no-op.  Tested
against existing configuration.

`configure-projects` uses the python-gitlab project to loop over all
projects in the instance and disable issues and wikis.

This also adds a requirements.txt and some rudimentary instructions in the
README.

Moves user-facing commands and configuration to top of repo, moves a few
common functions into a util.py.

Make `apply-settings` send JSON in the request body, which enables us to
set values that take an array.

Use that to handle restricted_visibility_levels and import_sources.

This commit reflects the current state of production settings.

Bug: T284336

Remove reference to wmf-gitlab-tools.

This seems like a better user experience overall than getting sent with no
explanation to an IDP login form.