Integrate Looker validation in GitLab merge request

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• In Looker, when you make changes to a development branch, you can check if your changes pass validation (and the UI runs the validator somewhat automatically).
• As part of the development workflow, you don't want to merge any changes, unless those have been validated.
• So we should expose that validation back in the GitLab merge request, via the Looker API: https://docs.looker.com/reference/api-and-integration/api-reference/project#validate_project
• Essentially, this is replicating the functionality of blocking a merge from happening unless the pipeline passed. Instead of the pipeline, we check the Looker validation.
• Similarly, we can replicate MWPS, but with merge-when-looker-validates.