Add Dangermattic configuration

[iangmaia]

Description

This PR adds Dangermattic to this repo. My main goal was to add RuboCop, but I added some other basic checks as well.

Please let me know if it makes sense to add more checks seen in other repos (reference)

NOTE: I was unable to configure the Danger key as I don't have access to the repo settings; will add it as soon as I get the access.

Testing Details

The main validation is to make sure CI is 🟢.

To run Danger yourself:

1. Checkout this branch
2. Run bundle install
3. Run:

   DANGER_GITHUB_API_TOKEN=<github_token> bundle exec danger pr <PR_URL>

   You should get in the console the same errors / warnings reported in the PR.

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• [x] I have considered if this change warrants release notes and have added them to the appropriate section in the CHANGELOG.md if necessary.