CI/CD setup

[luispabon]

• Add concourse build files for backend
• Add concourse build files for frontend
• Add concourse build files for admin
• Publish coverage reports to codecov
• Ensure PRs are linked to success of the pipelines

These probably will require the most work as currently we're doing no deployments from concourse:

• Build & push containers to gcr on master merges
• Trigger deployments after that