The unique index doesn't work when the port is `NULL`. So use `-1`
instead for when using the default ports (80/443), as if we would use
the real ports, we could still have both 80 and 443 in the database at
the same time.
* add a class for checking pod connectivity
* extend pod model to handle new functionality
* add an admin frontend to list pods and re-trigger checks manually
* add a daily worker to run through all the pods
* add unit tests for most of the new code
