Captcha Keypad 2.0.1

A housekeeping release. No change to how the keypad behaves.

The help page was showing the raw README file rather than written help; it now
explains the module and each of its settings, and links the configuration form.

The README described a much older version of the module and has been rewritten
to match 2.0.0, including an honest note that the keypad has to be clicked and
so is not suitable as the only challenge where keyboard-only or screen reader
access is required.

Testing: the Javascript tests can now be run locally rather than only in CI,
CaptchaIntegrationTest skips instead of failing when the Captcha module cannot
be installed, and CaptchaKeypadContactTest no longer assumes Contact is in
core. The stale CircleCI configuration is removed.

Issues #3618825, #3619044.