Upgrade ESLint configuration for ES2015 (without classes)

[colinrotherham]

This PR updates our ESLint config as part of #3463 to:

1. Use ES2015 (or ES6) as our target
2. Use arrow functions to preserve this in components
3. Use template strings rather than A + 'to' + B concatenation
4. Use let or const with block scoping rather than var

I've left ESLint rules in place for .prototype inheritance as we'll move to ES2015 classes in another PR

Browser support

We may go higher than ES2015 once we confirm the browser support guarded by our “cut the mustard” checks

We know that browsers supporting <script type="module"> already support the new features above

supports es6-module
not supports arrow-functions
not supports template-literals
not supports let
not supports const

[colinrotherham]

For all these arrow function changes I'm aware they're split into:

1. Arrow functions used to preserve this as the component instance
2. Arrow functions wrappers to avoid .bind(this) in event handlers
3. Arrow functions simply for "code style" or consistency

We have plenty of event listeners and .forEach() loops (like this one) that never actually refer to this so we may consider it unnecessary to change them to arrow functions outside of component scope

Terser is going to convert all that it can to arrow functions anyway to save KBs