[CI] Enable review apps

This enables review apps.

In MRs, the resulting AlekSIS version is deployed to our Kubernetes cluster and is made available via HTTPS automatically. It can be used for testing and demoing purposes.

(The superuser password is auto-generated and dumped as a build artifact.)

Later, we will integrate the Visual Review feature, allowing reviews to be made from within the deployed AlekSIS.

Currently, the review app deployed in this MR does not work because of !532 (merged). We can merge that first, then rebase this MR to see review apps start working.