Configure logging for the Cylc UI Server app

[kinow]

I added some logging in main.py some time ago, while working on the initial prototype. But there was nothing important in those logging messages.

While looking at #65 , I wondered where exceptions were logged. Looks like the GraphQL client code is not logging exceptions (neither is the rest of the application to be honest).

We need to look at how JupyterHub and its spawned applications handle logging, and use it properly in Cylc UI Server. A quick look at the JupyterHub code, looks like they use some code from Tornado logging.

But I am not sure if that's the only way they are handling logging.

Cheers
Bruno