Enable color for CylcHubApp in logs

[MetRonnie]

Different severity levels are all logged with the same colour in the logs, would be good to enable colour so that warnings and errors stand out.

[oliver-sanders]

For single user setups, people should use cylc gui. For multi user setups, cylc hub is required.

You are using JupyterHub for a single-user setup. Because CylcHubApp logs are also written to stderr they appear interspersed with the JupyterHub logs when JupyterHub is launched interactively from a terminal. This is not how it would work for a real deployment so this is only a concern for cylc hub developers.