Rewrite service to use Flask

[vpetersson]

Let's rewrite the diagnostics page to use Flask, which gives us a lot of free features out of the box. We should then serve this app using Gunicorn.

To keep things simple, let's ditch the Nginx container for the time being and serve Gunicorn on port 8000 with Docker exposing it on port 80. That way we don't need to run the container as root.

This is a per-requisite for #39.

[ryanteck]

@vpetersson Primarily the reason the container is run as root is to access many of the hardware and networking services. Not for Nginx.

[robputt]

@vpetersson I'd quite like to claim this one, after looking at the code base + tests in the outstanding PR the conversion should be relatively straight forward.

[shawaj]

Closed by #92