Add script to delete dangerbot comments

What does this MR do and why?

Give it a project ID and it will delete all Dangerbot comments on open MR's that are not associated with the latest Dangerbot user.

Usage:

ruby ./scripts/clear-dangerbot-comments.rb

Relevant slack thread: https://gitlab.slack.com/archives/CETG54GQ0/p1646338038256549

This script can be executed with a a Maintainer level API token for any project that is encountering 403 errors from previous Danger comments.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

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