Put a reverse-proxy in front of the voila server to give us more flexibility

Closes #10 (closed), and allows us to access the expert app at cara.web.cern.ch/expert-app instead of https://cara.web.cern.ch/voila/render/cara.ipynb.

This will be essential in the move to make the COVID calculator a separate REST service, and gives us full control of how our URLs look.

This also adds the ability for any branch to be pushed to the test-cara.web.cern.ch instance, to allow us to see complex infrastructure changes (such as this one) before pushing to cara.web.cern.ch.

Note that the CI pipeline only triggers a rebuild of the app image currently. It makes sense to upgrade this to rebuild all images when a change is made in the future.