Decide on ESLint rules

[gavinorland]

As per my Slack message, here's a commit showing the Airbnb ESLint overrides currently required for Frederic. Maybe we could each take a look through the rules - which sometimes have various sub-options - and note our preferences.

Then, we could comment in this thread or get together in person and decide which overrides we're going to keep, otherwise tweaking code into alignment. Personally, I generally go along with most Airbnb suggestions.

When we've agreed our preferences we can use that config for all repos.