We've been gradually converting anonymous functions to arrow functions
since adopting ES6. This commit uses ESLint to finish the process by
converting the remaining non-named, non-`this`-using callbacks to arrow
functions.

 - Enable `prefer-arrow-callbacks` lint rule. The `allowNamedFunctions`
   option is enabled because we have a few instances (eg. in
   gulpfile.js) of explicitly naming functions for use in debugging /
   logging etc.

 - Run `eslint --fix .` and `yarn format` to automatically fix up the
   code