Upgrade eslint-config-galex

Galex has changed its API significantly: https://github.com/ljosberinn/eslint-config-galex#examples, so I've migrated to the new syntax. There's a couple of new linting errors too, which I've attempted to fix (see comments).

After pulling the changes, don't forget to run npm install and to "Restart ESLint server" in VSCode. If you get an ESLint error when running npm run eslint saying that some rules aren't available, please run npm un react-scripts && npm i --save-dev react-scripts. This will ensure that ESLint finds the latest versions of all the linting plugins (instead of the old versions installed by CRA).

I've also configured NPM (via .npmrc) to specify exact package versions in package.json when installing new packages (instead of ^).