Add webcompat core countries ranking

[jgraham]

Checklist for reviewer:

• Commits should reference a bug or github issue, if relevant (if a bug is
  referenced, the pull request should include the bug number in the title)
• Scan the PR and verify that no changes (particularly to
  .circleci/config.yml) will cause environment variables (particularly
  credentials) to be exposed in test logs
• Ensure the container image will be using permissions granted to
  telemetry-airflow
  responsibly.

Note for deployments: In order to push images built by this PR, the user who merges the PR
must be in the telemetry Github team.
This is because deploys depend on the
data-eng-airflow-gcr CircleCI context.
See DENG-8850 for additional discussion.