feat(slack): onboard a site to ESS

[ekremney]

Please ensure your pull request adheres to the following guidelines:

• make sure to link the related issues in this description. Or if there's no issue created, make sure you
  describe here the problem you're solving.
• when merging / squashing, make sure the fixed issue references are visible in the commits, for easy compilation of release notes

If the PR is changing the API specification:

• make sure you add a "Not implemented yet" note the endpoint description, if the implementation is not ready
  yet. Ideally, return a 501 status code with a message explaining the feature is not implemented yet.
• make sure you add at least one example of the request and response.

If the PR is changing the API implementation or an entity exposed through the API:

• make sure you update the API specification and the examples to reflect the changes.

If the PR is introducing a new audit type:

• make sure you update the API specification with the type, schema of the audit result and an example

Related Issues

Thanks for contributing!

[solaris007]

tests missing

[github-actions]

This PR will trigger a minor release when merged.