Add formatting for Nunjucks files

[rmccar]

What is the context of this PR?

Fixes: #3169

Adds the Jinja plugin (prettier-plugin-jinja-template) to prettier and have then run prettier on all our Nunjucks files to format them correctly. I have also added them to the lint staging step so that they are formatted on the Husky pre commit hook. I have also fixed all errors and warnings from running prettier which has required some refactoring.

Some other fixes have gone in too:

• Removed some component imports in some examples because those components weren't being used.
• Removed the setting of the variants param on the related content component in some examples. This was having no effect because it's hardcoded to "bare" in the macro.

How to review this PR

• Tests pass
• Check through all files checking no unexpected changes have happened
• Check no breaking changes have been introduced

Checklist

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