Add ability to pass translation strings to accordion component via data-attributes

[vanitabarrett]

What

Adds ability to pass translation strings into the accordion component, using the Nunjucks macro.
Needs to work with the i18n logic added in #2801. There will need to be some kind of logic to look up the data translation attributes set on a component and return them as an object

Why

So users can translate the hardcoded strings in our accordion JS component
Passing translations via the Nunjucks macro enables them to do this without having to understand where different strings are (e.g: Nunjucks vs JavaScript) as it’s the same interface for both.

Who needs to work on this

Developers

Who needs to review this

Developers

Done when

• The accordion component has new Nunjucks macro options for passing in translation strings (following the format decided on in Decide JavaScript and Nunjucks API for passing translation strings #2780)
• Implemented JS code to lookup translations set using data attributes
• Translations provided via data attributes are in a format that the internationalisation logic understands, i.e: a JS object
• The accordion component JS uses the internationalisation logic to swap out hardcoded strings
• The accordion component still works if no/only some translations are passed (falls back to defaults)
• The accordion component has tests which cover the new ability to translate
• Added to or created a full page example which shows full translation of the accordion component