Make ES20xx feature use consistent across the project

[flaki]

Currently syntax-breaking ES6 features (like let or arrow functions) are usually only used on the backend, but the feature started to creep into the frontend world as well.

Make sure we use those features consistently across the project. It's clear that we want to use more ES6 across the whole of the project, but #6 will be needed for sure to be able to take advantage of the added features.