Ensure use of block body for arrow functions

DEFRA/water-abstraction-team#115

Just like [parens in arrow functions](#1004) how you write an [arrow function](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions) can differ.

```javascript
const materials = [
  'Hydrogen',
  'Helium',
  'Lithium',
  'Beryllium'
]

// Block body
materials.forEach((material) => {
  console.log(`${material} - ${material.length}`)
})

// Concise body
materials.forEach((material) => console.log(`${material} - ${material.length}`))
```

The _block-body_ version can be applied in all cases. But like with parens where devs will often drop the parens when there is only 1 param, they'll use _concise_ for simple arrow functions and _block-body_ for the rest.

To us though, the argument is the same. For those new or inexperienced with JavaScript this inconsistency can be confusing and a blocker to progressing with the language. The more we can do to keep the code base simple and consistent, the easier new folks will find working with it.

So, this change enables [the rule in ESLint](https://eslint.org/docs/latest/rules/arrow-body-style) that requires all arrow-functions to use the block-body.