Update `eslint` and `eslint-config-galex`

All right. So I have updated eslint and eslint-config-galex. This throws a lot of new warnings due to new rules in eslint-config-galex.

While I could disable them altogether, I think this is a great opportunity to revisit some of the existing code to improve it.

In this MR, I fixed warnings for a few files to show you a bit what is expected. We can discuss it together next week.