Change base paragraph size to be 16px and 18px in .editorial content

@moneyadviceservice/frontend 

Matt has decided to go with a base paragraph size of 16px (not 18px) unless the content is editorial (lives inside the .editorial namespace.

Pretty sure this may cause issues, so let me know :rotating_light: 
For readability I think this is a little clearer.

```css
.editorial p {
@extend %paragraph;
}
```