Add Continuous Integration using Github Actions

[peace-maker]

This builds the frontend and backend using Node.js 16, 18, 19, and 20 and checks if the database migrations apply on a fresh database. It verifies the generated graphql bindings are up-to-date too.

To allow to verify database migrations, I've added a DB_MIGRATE_ONLY environment variable, which causes the api to exit after processing the migrations.

In a separate step it runs linting steps as eslint and prettier to catch style problems.

We could run docker compose build as well just to verify the docker setup works? I didn't go for this in the first place, since you don't have yarn in the frontend container anymore to verify the codegen.