Add rewrite rule for legal fonts

[tbille]

Done

Fixes https://github.com/canonical-web-and-design/design.ubuntu.com/issues/158

QA

• DOCKER_BUILDKIT=1 docker build --build-arg BUILD_ID=test --tag design.ubuntu.com .
• docker run -ti -p "8006:80" --env SECRET_KEY=secret_key design.ubuntu.com
• Make sure that http://localhost:8006/font/ufl redirects to https://ubuntu.com/legal/font-licence

[webteam-app]

Starting demo at: https://design-ubuntu-com-canonical-web-and-design-pr-159.run.demo.haus/