Upgrade ESLint configuration

[romaricpascal]

What

Update our ESLint config to reflect the way that we write JavaScript now that we no longer support IE8-11.

For example, re-enable the no-var rule and update the rules for the es-x plugin.

Fix any linting errors that occur. For example, replace var with let or const as appropriate.

## Why

With our new strategy for browser support and us dropping IE, we can write more modern JavaScript

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• ESLint configuration has been updated to match how we write JavaScript
• JavaScript errors have been fixed