Improve logging

[pokey]

I believe we've now switched all logging to use console.debug, but we should still do the following:

• Understand what determines whether console.debug appears in the output
• Add a setting to turn debug logging on and off
• Add a linter rule to ensure console.log isn't in the code anywhere
• Look into a more full-featured logging solution, eg so that we can turn logging on only for certain parts of the code / use log levels

[pokey]

Partially solved by #380