Configure AssetHelper to load individual stylesheets

[jon-kirwan]

What

https://trello.com/c/9RMdBVHj/1866-enable-individual-loading-of-stylesheets-in-collections

Changes to load component and view style sheets only required on the page being viewed. For example, CSS for "topics" only loads on the following pages:

• https://www.gov.uk/topic/oil-and-gas
• https://www.gov.uk/topic/oil-and-gas/environment-reporting-and-regulation
• https://www.gov.uk/government/organisations/hm-revenue-customs/services-information

Why

This reduces the amount of CSS required for each page and increases the ability of a browser to cache the stylesheets - this should mean a faster load time for both first time and repeat visitors.

See RFC #149 for more details.

Review URL(s)

• Browse page
  • A - Z page - Armed forces
  • Curated page - Manage an existing benefit, payment or claim
• Coronavirus (COVID-19)
• Cost of living
• Organisations homepage
  • Prime Minister's Office, 10 Downing Street
  • Cabinet Office
  • HM Prison and Probation Service
  • Civil Justice Council
  • Commercial Court
• Past Chancellors of the Exchequer
• Past Foreign Secretaries
  • Edward Frederick Lindley Wood, Viscount Halifax
• Past Prime Ministers
• Services and information page - Services and information - HM Revenue & Customs
• Step by step page - Learn to drive a car: step by step
• Taxonomy
  • Taxonomy page - Education, training and skills
  • Worldwide taxonomy page - UK help and services in Japan
  • Secretary of State for the Home Department
• Topic page - Oil and gas: detailed information
  • Subtopic page - Onshore oil and gas sector guidance
• Topical events:
  • First World War Centenary
  • Her Majesty Queen Elizabeth II
  • Russian invasion of Ukraine: UK government response
• World location news - Japan and the UK

Visual changes

None

Anything else

• application.scss imports custom margin helpers /helpers/_margins.scss which I think could be replaced with Design System utility classes (with some very minor differences).
• application.scss then only includes the following settings (below) which can be removed since they are included with _individual_component_support.scss (imported from govuk_publishing_components) and application.scss can then be removed

$govuk-new-link-styles: true;
$govuk-include-default-font-face: false;

• Set up gem so stylesheets can be loaded individually govuk_publishing_components#3014
• Set up individual component CSS loading frontend#3342
• Set up individual component CSS loading smart-answers#6213
• RFC #149

[KludgeKML]

Looks good to me.

[MartinJJones]

Nice work! The changes look good to me, any thoughts I had are already covered in the "Anything else" section you provided in the PR description 👍