Improve CI

[inyono]

If someone wants to learn how to define CI workflows with GitHub Actions: https://github.com/serlo/serlo.org-database-layer/tree/main/.github has some patterns that we can also use in serlo.org (for example: setting up a database and waiting until it's ready). So, we could possibly:

• Run e2e tests (might run into some limits here, not sure)
• Run contract tests (this won't be needed soon-ish since the routes are moved to database layer)

[elbotho]

Since this repo is EOL we won't work on this.