Configure AssetHelper to load individual stylesheets

[jon-kirwan]

What

https://trello.com/c/ws6n64fm/1870-enable-individual-loading-of-stylesheets-in-email-alert-frontend

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

• https://www.integration.publishing.service.gov.uk/email-signup/?link=/money

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)

• https://www.integration.publishing.service.gov.uk/email-signup/?link=/money
• https://www.integration.publishing.service.gov.uk/foreign-travel-advice/canada/email-signup
• https://www.integration.publishing.service.gov.uk/email/subscriptions/new?topic_id=statistics-with-1-research-and-statistic-5e2982632b
• https://www.integration.publishing.service.gov.uk/email/subscriptions/single-page/new?topic_id=pro-innovation-regulation-of-technologies-review-life-sciences-interim-report
• https://www.integration.publishing.service.gov.uk/email/manage
• https://www.integration.publishing.service.gov.uk/email/manage/address (if no "change your sign in details in your GOV.UK One Login" message)
• https://www.integration.publishing.service.gov.uk/email/manage/authenticate
• https://www.integration.publishing.service.gov.uk/email/manage/frequency/dd1e3465-6598-4c72-b7ac-94f785164ae6
• https://www.integration.publishing.service.gov.uk/email/manage/unsubscribe-all
• https://www.integration.publishing.service.gov.uk/email/unsubscribe/dd1e3465-6598-4c72-b7ac-94f785164ae6
• https://www.integration.publishing.service.gov.uk/email/subscriptions/verify

Visual changes

None

Anything else

• I've removed application.scss from the manifest and current layout since it no longer imports any partials and compiles to an empty style sheet. I think this is a good approach since developers can still find the expected application.scss file and read the comment as to why it's empty / not loaded.
• 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

[MartinJJones]

Changes look good to me 👍

[KludgeKML]

Yeah, this all looks okay to me!