Update to newer Gov.UK Design System to comply with Service Standard

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Currently Hof uses the deprecated Gov.UK Frontend Toolkit which itself does not meet legal WCAG 2.1 accessibility standards. The replacement Gov.UK Design System is necessary for new services in particular to meet:

1. legal accessibility standards
2. Align to the Gov.UK Browser compatibility list
3. Incorporate newer frontend components and patterns.

This issue is linked to #167 Update Error summary as per GDS Design System.

The Design system has a number of changes from the older Gov.UK Frontend Toolkit. The migration guide is documented here. Notably:

• Newer style sheets, JS, and assets
• CSS styles are prefixed with gov
• Use of Nunjucks for templating. Appreciate Hof uses Mustache