database migrations should be a pre-deployment hook
    
    ... not a post-start pod hook for the web app. This way, we can see the
    logs of the migration pod, and if the migration fails the deployment
    will be aborted.
    
    The database migration pod has to first wait for the Postgres pod to
    finish starting up, because in a freshly created environment (as in the
    tests) the migration pod is started concurrently with the Postgres pod.
    
    Fixes #121.